Bill Ball

Yes, many people have done a lot of manipulations. It's been pressure bled a lot at 15 PSI, reverse bled at 20 PSI, pedal bled, bled while cracking the fittings at the master and ABS unit, bled while activating the ABS solenoids, bled with the calipers held up in the air and banged on to eliminate the last hump in the line and free any bubbles that might be trapped in the caliper bores, bled front brakes first many times. How it was bled right after the new master was installed is not clear to me.

jon928se

I'm not sure what effect the check valves in the pistons have on that statement - I need to clean the pistons up some more before I put them in my mouth and blow through them. The valve in the rear circuit piston is mechanically held in "a " position when the system is at rest.

RHD cars are the same as LHD cars in this respect, the RF is furthest away so bled first, as the lines to the calipers originate at the ABS unit which is on the LHS on all S4+ 928s.

As with the front calipers RHD order should be the same as LHD. The feed to the Tee piece on the rear cross member on both LHD and RHD is down the LHS (Under the US drivers position) of the car. Right Rear is furthest from the ABS unit and the Tee piece by about 3 feet of brake pipe. (The bastard of a pipe to replace as it goes over the top of the tranny.)

I'll take some more pics of the MC internals and post them here when I've got dressed (just got out of bed and been under the GT in my dressing gown already)

dr bob

Jon--

Going by the instructions that came with the replacement MC of similar construction. The instructions came written from the mfr and verbally from the vendor. Apparently they get back too many 'defective' rebuilts if the bleed instructions aren't followed carefully.

Thanks for the correct details on the LHD. Didn't suspect that Porsche would plumb all the hydraulics across to the left fender, but I guess it was simpler/easier than tooling for ABS where the horns and electricl harness live there on the right. I've seen pictures of cars that are on the wrong side of the steering wheel, and Iwatched a guy work on a Fiat transmission once. Qualifies me as a master mechanic these days.

jon928se

Wasn't doubting your original statement. More admitting my own incomplete (at this time) understanding of how the system works!

jon928se

Some observations following rebuilding the brake MC and Clutch SC.

Bleeding order
MC, Clutch, RF, LF, RR, LR
Calipers - Inside bleeder 1st

Using pressure bleeder at 22psi starting pressure, (pressure comes from connection to tyre valve), bled approx 1.2L of fluid into and through the system (more than needed to get rid of the bubbles but was changing fluid to a diff colour)

After this stage I was reasonably certain that no air in the MC or in the pipework.

Left system pressurised for a day and half.

Did a quick bleed at all points again to check for bubbles - none. about 0.1 L of fluid.

Then did the "wife holding brake pedal down with both feet" open bleeder pedal drops to floor, hold it, close bleeder, let pedal up slowly - repeat, 3? times or until no bubbles method. System still pressurised at ~ 22psi.

On the RF inside caliper - 1st one to be bled - got bubbles out at 3rd, 4th ( lots), 5th (lots) 6th (few) 7th (no bubbles) repetitions. moved on to the remaining calipers but no bubbles of significance.

My thoughts - I just got the air out of the ABS unit. - I will repeat the procedure tommorow leaving the system pressurised in the interim.

Pedal engine not running seems harder/higher to operate.

Other observations - when doing the rear calipers by this method the brake pedal does not go as far down as it does doing the front calipers ????

SharkSkin

22 psi on a 5-speed? Jon, you're braver than I am, unless you clamped the ends of the blue hose...

Interesting results, We didn't leave Nicole's pressurized that long, but we could. It's not good for the nerves, knowing that there is a risk of spewing brake fluid onto paint.

It will be very interesting to hear how yours feels on the road when you are done.

BTW, the reason the pedal doesn't go as far down when bleeding the rear is, the rear chamber on the MC(front brakes) is a larger bore than the front chamber, and there is a floating piston between the two which only moves via hydraulic pressure from the rear chamber(front brake circuit). When you bleed the fronts you empty that chamber to the end of the stroke. When you bleed the rear, you empty the smaller bore as the floating piston pushes forward, but because of the larger volume of the rear chamber it doesn't have to go as far to do so. There is a cross-section in the ABS tech doc that illustrates the layout pretty clearly.

jon928se

New Blue hose clamped both ends and new reservoir seals. - Re leaving it pressurised for a while - no real dramas - I figure if it's going to pop the reservoir off it'll do so pretty early - it hasn't moved yet. Anyways the pressure is probably down a bit now as you loose some every time you disconnect to top up the remote reservoir.

I have no idea if leaving it pressured up does any good - more just laziness not undoing it. Same reason for leaving it a day or so then having another go - PITA taking putting the wheels on and off etc. And I've got other jobs I need to like removing the upper A arms and putting bigger washers on the inboard end of the PU bushings - current ones are trying to work their way into the bushes.

The highest pressure I've used was about 36 psi on a car I bought very cheaply that had sat on the street for a few years and then in the pound for 5 or 6 years - only way to clean out the system - pressure it up then remove a bleed nipple completely - once it's squirting high in the air the system is clean.

LeoDioxide

FWIW, when I finally got decent brakes from bleeding 2-person method, the car was on a slight inclined surface (maybe a couple of degrees) and had just come off of jack stands, meaning the nose was tilted up.

jon928se

Update

I redid the hold the pedal down then crack the bleeder valves method yesterday and got a little more air out of the front right. Put the car back on it's wheels and went for a short test drive......

Pedal is still uninspiring (compared to my 88SE) but is about the same as where it was after the last fluid change two years ago. (When all the fluid in the system was emptied) With cold (air temp about 20C) tyres I can make the ABS work on a decent road surface - all 4 wheels , but the pedal travel is too soft and too long.

I was going to have a look at the pedal pushrod location to check if there was any preload but pulled muscles prevented the contortions neccessary to do this. However I do recall (from previously overhauling the pedal system to cure a squeaking clutch pedal) that the brake pedal doesn't have a physical "stop" other than the brake light switch which is adjustable. Which effectively means that you can set the brake pedal height then set the preload on the pushrod - doesn't seem right to me.

Not knowing if my cleaned up but no seals replaced M/C could be the cause of the problem I am going to buy a LHD M/C and convert the pipework to fit - (Can't justify the US $550 for a RHD M/C when a LHD is $125) to eliminate one possible issue. I will probably reverse bleed after installation to avoid blowing more air through the system as I will loose some fluid out of the lines from the MC to the ABS unit.

Nicole

Thanks for the update!

Bill and I tried the bleeding with the front of the car lifted up, and we got a few bubbles on the right side where we had put the old caliper last time (to see, if it makes any difference - it didn't).

jon928se

Some more update observations

Back end of last week i did two complete rounds of reverse bleeding. Used my pressure bleeder but connected to the caliper using a short length of double flared 3/16th copper and an M10x1 Tube nut with the feed from the pressure bleeder on t'other end of the short length of copper. Removing the bleeder and installing the copper+tube nut doesn't risk air ingress as the gravity pushes fluid out of the caliper as soon as you remove the bleeder. Do have to wait about 5 minutes after connecting the pressure bleeder so that air in the feed line is pushed out before applying pressure.

Using about 25psi I reverse bled through about 1/3 litre of fluid per caliper and repeated (By now I'm reusing the fluid which has only been in the system a few weeks.)

It's really difficult by yourself to see if any air comes out of the MC. I'm using the cap normally used to pressurise the reservoir connected to my collection bottle so the air (if there was any) bubbles will only show in the reservoir not in any visible pipework.

I haven't driven the GT yet but the pedal feels a little firmer (but not like I want it) - just put it back on it's wheels - but I hope to be able to get it inspected tommorow am and re-registered as I'm on a flight back to the UK tomorrow pm for at the moment an indeterminate length of time.

Nicole

Thanks for the update!

My car is still in the shop, getting a new brake bias valve now. I still hope that'll solve the issue; otherwise I won't take it to the Sharktoberfest.

Nicole

Good news: I visited my baby at the RennShop tonight, and got to drive it up and down the street.

After a thorough inspection, Marvin had replaced the brake bias valve, and then vacuum bled the system (three times, starting at the master cylinder, but he said there was no significant bubbles the third time).

When I drove the car, the brakes felt totally transformed. While I still feel the pedal travel is rather long and the pressure point a bit soft, the brakes respond like never before.

I did not have to push very far to get a response, and it was possible to engage the ABS without crazy stomping on the pedal. Deceleration felt better than ever.

So, I am very optimistic now. We discussed the pedal feel, and since we had not kept track of the positions of the pads before we removed them for the initial rebuilt, there is still a possibility that the feel might firm up when the pads are bedded-in again.

We agreed that the car is safe to drive to Sharktoberfest, and that we should see, if actually driving it (what a concept!) will firm up the pedal feel a bit.

The other good news: The bill came-in under the estimate. Awesome!!!

Mrmerlin

Nicole,.............. Motor!
Thats great news, I hope the brakes work well, and you can start enjoying your car

jon928se

Excellent news Nicole. lets hope they improve even more when the pads bed in.

Did your mechanic think the bias valve was part of the problem ? or an unrelated issue ?

update on the GT - it passed it's rego inspection pulling 1.27G max decelleration in the brake test. but the tester commented that he didn't think the brakes were particularly inspiring - I agree.

When I get back to OZ whenever that is I'll have to try vacuum bleeding - I have a mity vac but it may be time to buy one of those air powered venturi vacuum pumps.