No_snivelling

Interesting. Anyone else in the same boat? I did read somewhere that the filter will introduce air bubbles when adding fluid and should be removed when adding fluid which of course defeats its purpose. It was also argued the need for the filter in the first place; Clean fluid from a new bottle of brake fluid should not need filtering.

I wonder if it also acts as a guide and support for the fluid level float. But if people are running without it and not experiencing issues then maybe I should stop worrying about it.

No_snivelling

Ok so I have gone back to the workshop manual and read and re-read and studied the whole section on bleeding. It seems clear to me that it is not necessary to bleed the accumulator (the bomb) or the solenoid valve block unless under specific conditions like component replacement has occurred.

So in an ordinary flush, why do most people bleed the accumulator and valve block?

It is my understanding that bleeding sufficient fluid from the differential locks should also flush the bomb and the valve block. Am I wrong? What am I missing?

JasonAndreas

More than likely and as is usually the case with Porsche engineering, owners & mechanics removed it without having any idea of its purpose. It was not installed on 964 with vacuum assist brakes, only the high-pressure hydraulic systems have it.

LM964

'Restrictor bush' is the name given to it in the parts list. Might be less of a filter and more of way of preventing anyone dropping or poking things into the reservoir. No distinction given between a C4 or not. Get the impression all models had this. No idea where mine went but no dramas though..

No_snivelling

Given how hard they are to remove, it does not surprise me that owners don't reinstall it.
I have looked at close up pictures of it and you can see a pretty fine mesh screen in the side openings.
My theory is that it was intended to serve multiple purposes:

As a guide to keep the float from too much lateral movement.
As a fluid level dampener to prevent "waves" from giving false readings to the float. The small openings along with the screens would restrict levels inside the bushing from sudden changes when hard cornering or braking is taking place.
As a filter to keep users from inadvertently contaminating the reservoir.

Only some retired Porsche engineer knows for sure.

LM964

That to me makes perfect sense, given too how sensitive the C4 is to fluid level. These cars are race-bred, and track capable design thinking could have easily suggested a reservoir 'baffle'. Who knows..

JasonAndreas

LM964

Yup, my bad; i got as far as illustration 604-00 and straight to part #26 (which as a single part showed no model distinction) then ran with that rather than scrolling to 604-01. Thanks for pointing out.

No_snivelling

Hmmm. Ok there goes my theory that this device has anything to do with sloshing of brake fluid from hard cornering, or accel/decel since there should be no difference between a C2 or C4 insofar as sloshing is concerned.
But maybe the brake fluid level in a C4 goes up and down while the PDAS is in full swing and this device would smooth that out to prevent fluid level alarms? I am probably reaching here.

Or Porsche was worried about the diff locks screwing up if dirt got into the system from the reservoir. Any retired Porsche engineers listening?

Ok, I gotta stop hypothesizing and get out to the garage. The brakes are not going to bleed themselves!

No_snivelling

Does anyone know if the brake bleeding/flushing instructions in the factory workshop service manual were ever updated? I refer to the procedures as found in Chapter 4, pages 47-2 to 47-8.
Any TSB's or other updates?

No_snivelling

For what it's worth here is my experience and notes I made to myself to guide me along the way. Thanks for all the input and comments members have made over the years. It really helped!

Guide to Total Brake Fluid Flush for 964

Updated August 2021

Based on information gathered from Rennlist, Pelican, Adrian Streather’s Book, other internet sources and the Workshop Manual (WSM). See links below for greater detail, photos etc. This is more of an overview of steps and a list of some of the things I thought were important.

Disclaimer – Where sources gave conflicting or confusing information, choices had to be made. Please be advised that my choices may or may not be appropriate for you, I am by no means an expert Porsche mechanic.

I used the Motive Power Bleeder 100 (European) and the UDT999 scantool from Jenniskens. I used the power bleeder wet. It holds approximately 179 cubic inches = ~2.9 quarts or ~2.8 litres. Directions say to add maximum of 2 quarts so that means 70% full. 70% of 2.8 is 1.96 litres.

So, add no more than 2 litres to the power bleeder jug. Each bottle of Prestone DOT4 brake fluid is 946ml so 2 bottles is safe. If running the Motive dry (no fluid in the jug), then be warned it takes 400 hand pumps just to reach 12 PSI. The WSM says to use 22 PSI but I did not dare go that high for fear of cracking my 29 year old plastic brake fluid reservoir. I used around 18 PSI.

In general, the sequence to be followed is:

1) Bleed brakes and clutch first.
2) Hydraulic Boost System (Accumulator and Solenoid Block) (according to WSM this can be skipped if no components replaced and no air in system) but most people do this anyway and so did I.
3) Lateral differential lock cylinder
4) Transverse differential lock cylinder

Adrian’s Bleeding Procedures can be found at:
Brakes (page 267-268)
Clutch (page 268)
Hydraulic Boost System (page 265-267)
Differential Locks (page 269-270)

WSM covers bleeding in Chapter 4 page 47-2 to 47-8.

The WSM does not refer to the lock cylinders with consistent terminology. They alternate between Transverse, Lateral, Axial and Longitudinal. I am assuming that:

Transverse = Lateral
Axial = Longitudinal.

Many sources state that in any total bleed, the accumulator and solenoid valve block must be bled. The WSM however states this only needs to be done under specific circumstance such as when components are replaced. I am going to assume the WSM is correct, but the manual would be based on a fluid purge every two years so I am going ahead with it. If in doubt about when the brake fluid was last changed, it is probably wise to flush the accumulator and valve block. Otherwise, you are pushing all the old fluid in the accumulator and valve block through the differential locks which would be slow and might allow dirt to get trapped in the locks.

Dlpalumbo said “The clutch fluid pickup in reservoir is much higher than you'd guess by a quick look at it. I tried a dry bleed at first and got air in system, peddle went to floor. Reverted to wet bleed and all is well.” He also said “I would think if you bleed one nipple at a wheel, the second will take much less as the line is already bled. So, the split would not be 50/50 as suggested. I don’t know how much the first nipple will take though. Also, the right rear should require more fluid than the left front. The 250 cc probably includes chamber in reservoir, which may be the largest part of the total.”

I bled my brakes with the following steps and quantities:

Clamp overflow hose, connect power bleeder and pump-up pressure to 15-18 PSI. (WSM says 22 but with an older plastic reservoir, I did not want to push my luck. Also, the spec for the Tygon tube that comes with the Motive bleeder says maximum working pressure is 20 PSI. This comes from the manufacturer of the tube. Better safe than sorry is my motto. And in that light and because I am super cautious, I covered every painted surface anywhere close to the power bleeder with a sheet of poly.

Bleeder wrench size for brakes is 11 mm.

All 4 wheels – I decided to take 175 cc on outer and 100 cc on inner bleed screw (total of 275cc) WSM says to take 250 cc at each wheel. This is likely so the reservoir and system is totally purged. In other words, they want you to flow 4 x 250cc = 1,000 cc of brake fluid through the system. I flowed 4 x 275 = 1,100 cc to be safe. Plus, I used a turkey baster to remove about half or more of the reservoir fluid to start. This will help with a complete purge of old fluid. Probably overkill but better too much than too little.

It helps to tap calipers while bleeding with a soft face hammer. This helps in removal of small air bubbles that may be trapped in the caliper.

Tighten brake bleeder screws (m10-1) to 6-9 ft/lbs or 8-12 Nm if you are not comfortable with torque by feel.

Apanossi said: “Three out of 4 calipers had slow draining from the outermost nipples. I'm assuming that this was due to sludging once again from infrequent changes. To overcome this, I unscrewed the nipple completely under pressure from the bleeder and stuck a 14 gauge copper wire down the channel to free up any loose sludge. If this still did not produce adequate flushing, I then gently stepped on the brake pedal (bleeder still pressurized) and that caused brake fluid to squirt out and produce a nice flow. Keep the opening covered with paper towels to avoid it squirting on the body. The nipple can then be screwed back on.” Note - instructions say not to press brake pedal while using Motive under pressure or damage to master cylinder may occur, but Apanossi appears to have not suffered any damage.

Ignition must be off and remain off for the entire bleed of brakes and clutch circuits.
Brake pedal must not be pressed any time power bleeder is in use. Master cylinder could be damaged.

Clutch is 7mm bleeder and should need about 50-100 cc of fluid. Clamp reservoir overflow hose and connect power bleeder and pump-up pressure to 22 PSI max. I used 18 PSI. Be very careful if using Motive dry because reservoir barely has more than 100cc of capacity. Very easy to run it dry and get air into the system. Also due to limited wrench space, it might take two swings of the wrench to get the bleeder open enough for it to flow.

Procedure for accumulator/valve block if components opened or replaced or if air in system or if you want a total system purge:

Accumulator Bleed/Valve Block Bleed: Need 9mm and 11 mm wrenches.

This section follows the WSM procedure.

*Read notes at bottom first.
Check for lots of fluid in reservoir.
Ignition off and then disconnect electrical power to pump.
Clamp overflow hose on reservoir.
Connect power bleeder and pump to 22 PSI Max. I used 15-18 PSI.
Do not touch the brake pedal at any time the bleeder is in use.
Depressurize by pumping brake pedal about 25 times or by opening accumulator bleeder screw after connecting collection bottle to accumulator bleed screw.
Open accumulator bleed screw.
Turn ignition on.
Connect plug to pump and watch fluid until no more bubbles.
Disconnect plug and close bleeder.
Check fluid level.
Open bleeder valve on valve block
Connect plug to pump and watch fluid until no more bubbles.
Disconnect plug and close bleeder.
Reconnect plug and allow pressure to build up until pump shuts off.
Disconnect pump plug.
Hold accumulator bleeder tube firmly and open accumulator bleeder. Caution. High pressure.
Completely drain accumulator and tighten bleeder screw.

Repeat this procedure until no more bubbles. Complete fill and complete drain off accumulator pressure one or two times.



*Notes:
Goughary says “First- hook up a bleeder bottle and then make sure that when you open the bleeder on the accumulator, that you do that slowly. If you do it quickly, you will blow the bladder inside the accumulator.
Once the pressure is released, turn the key to "on" so the pump is pumping fluid through to the bleed bottle. Motive obviously has to be connected and full. Run a half bottle or so through...then close the nipple and let the system pressurize. Once it's fully pressurized, crack the bleeder just a little, and let some pressure out but *don't release all the pressure. Close it and pressurize, then do it again. Repeat till no air.” (edited to clarify instruction)

*Notes:
Dfinnegan suggests to let the fluid sit in the charged bomb for 20-30 minutes between the final two bleeds to allow the fine air bubbles to settle out for the final bleed.

Solenoid Valve Block Bleed:
Exact same procedure as for Accumulator Bleed above.
Check for lots of fluid in reservoir.
Ignition off and then disconnect power to pump.
Clamp overflow hose
Connect power bleeder and pump to 22 PSI Max. I used 15-18 PSI.
Do not touch the brake pedal at any time the bleeder is in use.
Depressurize by pumping brake pedal about 25 times or by opening valve block bleeder screw and connecting collection bottle to valve block bleed screw.
Open bleeder on valve block.
Turn ignition on.
Connect plug to pump and watch fluid until no more bubbles.
Disconnect plug and close bleeder.
Check fluid level.
Open bleeder on valve block
Reconnect plug and allow pressure to build up until pump shuts off.
Disconnect pump plug.
Hold valve block bleeder tube firmly and open valve block bleeder. Caution. High pressure.
Completely drain valve block and tighten bleeder screw.
Repeat this procedure until no more bubbles. Complete fill and complete drain of valve block pressure one or two times.
Check tightness of both bleeder screws, disconnect power bleeder and unclamp vent hose.
Press brake pedal several times. (WSM does not say with ignition on or off)

It is mandatory that at this point the lock cylinders get bled as per below procedure.

I have found significant differences between various sources and WSM methods for bleeding of the diff locks. As an experiment I will favour WSM methods. Note- it worked for me.

After Brakes, Clutch slave and accumulator and valve block are bled, move on to diff locks:

Bleeding Diff Locks
This section follows the WSM procedure except I used UDT999 instead of the Porsche Hammer:
Need 7 mm wrench.
Check drain hole in the bottom of the diff locks. (see Goughary’s thread on this)
Ensure battery is well charged.
Check for lots of fluid in reservoir.
Make sure if pump electrical plug was disconnected that it has been reconnected.
Make sure reservoir vent hose is open.
Do not use power bleeder because brake pedal needs to be pressed.
Connect UDT999/PDT999/Hammer or other diagnostic tool.
Turn on ignition, select PDAS, Bleed of Transverse/Lateral Lock cylinder.
Unscrew bleeder on Transverse/Lateral diff lock cylinder and drain 250-300 cc Max.
Select Stop and Reduce pressure.
Add more brake fluid to reservoir.
Operate brake pedal several times with ignition still on.
Repeat with Axial/Longitudinal diff lock cylinder and drain about 200cc.
Disconnect UDT and operate brake pedal with ignition on until pump starts and then stops.
Correct brake fluid level. Do not overfill.


List of Helpful Links

https://rennlist.com/forums/964-foru...procedure.html



RicardoD
https://rennlist.com/forums/964-foru...-question.html

Utah914
https://rennlist.com/forums/964-foru...placement.html

Dfinnigan
https://rennlist.com/forums/964-1989...i-thought.html

Garrett376
https://rennlist.com/forums/964-1989...e-of-cake.html

Garrett376
https://rennlist.com/forums/964-foru...l#post15633352

Aldo964
https://rennlist.com/forums/964-foru...ocedure-5.html

RicardoD
https://rennlist.com/forums/964-foru...l#post16376569

Myles Maycher
https://rennlist.com/forums/964-foru...rake-pump.html

jhy
http://forums.pelicanparts.com/porsc...e-booster.html

edc153
https://rennlist.com/forums/964-foru...rblue-out.html

Metal Guru
https://rennlist.com/forums/964-foru...-bomb-how.html

Mark Shaw
https://www.pelicanparts.com/techart...eed_brakes.htm

Pelican DIY
https://rennlist.com/forums/993-foru...-abs-pump.html

skm
https://www.motor.com/magazine-summa...ble-june-1998/

RicardoD

This has come full circle for me because after I get the engine in the car, this is the first thing I need to do. I opened some brake lines changing the suspension on the car. I wonder if I can do the PDAS lock bleeds with the engine out of the car for easy access. I am scared to turn the key to the ON position with the engine out of the car.

I need to order more brake fluid, I think Goughary recommended 5 to 6 qts with the wet method.

Goughary

I use a much larger motive than the one they sell at pelican. This way i never have to add fluid. I do use typically 5-6 in the motive. But i also very often end up pouring a bunch back into a can.

One correction from above- i never suggested to not have a bleed bottle connected to the bomb when releasing pressure. Always have a bleed bottle connected to the bomb if you are going to open the bleeder. ----just open the bleeder slowly, or you may blow the bladder.

Love this thread. I do so much bleeding now that i have the whole fluid flush on a c4 down to 1.5 hours working slow and talking and futzing around the entire time...can be a fairly quick thing without distractions and futzing...

No_snivelling

I went through 5 liters but I think its a good idea to have at least one extra on hand. No idea about doing the locks with engine out, but at least you can make sure the bleeder can be easily opened and you can pre-attach the bleeder hose.
I found it really hard to attach the bleeder hose since you are working blind with only one hand. So you could attach the hose and a flare nut wrench with the engine out (that's the hard part done) and then after engine is in you just have to remember to finish this job before you take it for a spin. I refer only to the lock that is closest to the engine. I don't think the forward lock would be any easier with engine out.

DWS964

I always start a brake fluid change with the bleed/flush of the accumulator and locks solenoid. These actions consume a large amount of fluid, thus doing an effective job of purging the large reservoir of old fluid, prior to doing the clutch and brake systems. For this same reason, I do the clutch slave bleed next, to purge the clutch chamber in the reservoir of old fluid. Somewhere there is a good diagram of the chambers in the reservoir and their respective volumes. Now, when you start the caliper bleed sequence, you are assured of a supply of fresh fluid to purge the master cylinder and lines to each caliper. I do the entire system every time I do a fluid change (1-2X per year, depending on how much track time I have). The accumulator bleed is very accessible and easy, and it only costs an extra liter of brake fluid, so why not do it? Take a look at how much the reservoir volume changes when you pump the pedal to release accumulated pressure, and then how much it drops when the pump runs through a full pressurization cycle. Why mix that much old and new fluid whenever the accumulator depressures? Our aging accumulators don't hold pressure like they did when new, when the car sits for a few days. Does your pump run for a couple seconds on startup, or take a couple minutes (and triggers the warning chime)? And, do you hear the locks activate when you first turn on the key before starting the engine?
It was great to see your deep dive into this procedure.