dfinnegan

The pressure switch on my hydraulic pump went a couple of weeks ago. I figured I'd post the fact for statistical reference and provide some background on what happened, what the fix was, and what I learned along the way.

The biggest thing I learned was that I was lucky! The fluid ran out fast. I barely used the brakes at all and the pressure went out. This could have been very dangerous. If the lights go on, turn off the car!!

Thankfully I was very close to home and not driving fast. I've covered ~2500 miles in the past few weeks. Thank goodness I wasn't 300 miles from home or at highway speeds.

I ran out for an errand and within 1/4 mile from home the Master Alarm light, 4WD System light and the Brake Pressure light all came on. I stopped the car and upon restart the lights went out, but came back on as I pulled away. On returning home I found a large brake fluid spill in the driveway where the car had been parked. After pulling it into the garage the leak persisted, creating a large puddle. More than half the brake fluid reservoir emptied out before the leak stopped.

The leak came from the joint between the pressure switch and the pump. From my research here that indicated either the seal was gone or the switch was bad. By the volume of the leak I expected that the switch was bad and ordered a replacement.

Having installed the replacement all is well again.

The job required removing the pump by first removing the pressure from the system by pumping the brake pedal 20-25 times with the car off. The brake pedal becomes very stiff. Then removing the hard line with an 11mm brake line flare wrench (high pressure output to bomb) and the soft line by pulling the cotter pin (low pressure input from reservoir) and, finally, removing the single bolt that holds the pump in place (not the pump mounting bracket). On the work bench I was able to remove the switch using a large crecent wrench. I crinch to even think about it, but I was not up for the $150 "special tool" which is just a 36mm deep socket with thin walls (45mm O.D. max to fit). I tried a couple of standard 36mm sockets, but they would not fit between the switch and the pump.

Upon re-installation I bled three full quarts of brake fluid through the entire hydraulic system: the bomb (high pressure reservoir), lock unit, tranverse and longitudinal locks, 8 brake nipples and the clutch. I bled the bomb 3 times; with one bleed going too low on the master brake fluid reservoir; thus requiring the extra bleeds. I 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.

Interestingly, having done, what I believe is, a full and proper bleed on the bomb, my pump run-time when first starting the car is now considerably shorter! What used to be 30-40 secs is now barely 10 secs. Perhaps long run-times are assoicated with air in the system. Perhaps it was the switch going bad.

Some useful information:

Wachuko

Thank you. I was new to the 964 when it happen with mine and had the dealer do it.

Had a similar situation back in June 8, 2000.

I was using the car and had to make a sudden stop
at a speed bump. The ABS came to play. Since it was the first time I had sense the system in action I decided to give it another try. It worked flawlessly. Close to my house I made a normal stop and to my surprise the brake pedal would not respond. I had to push it extremly hard to make the car stop.

As in you case, the problem was solved by: Pressure Sensor replaced(US$247.55 - Part # 964-606-904-00), Relay replaced (US$33.81 Part # 964-615-135-00), Rubber O-Ring replaced(US$4.99 Part # 999-707-094-40). Labor US$147.90 Minus PCA discount. Those were dealer prices since I did not know about Sunset Imports/Porsche at the time... looking at what you paid it had to be a heck of a lot cheaper back in 2000.

Makes me wonder if this will be one of those wear items that we should have a spare just in case... if prices keep going up it might just be wise to stock one of these in the spare parts bin.

Thanks for the write up!

Feudal Serf

reviving an old thread cause it helped me.

I was troubleshooting some suspected ignition issues after a top end rebuild.
Dash instruments fuse went due to my fiddling with the ignition coil harness.

I figured out that the lower wires were routed incorrectly and after fixing this i had the PDAS and Brake light.

1990 C4

The pump started and system was pressurised and pump shut off.
Brakes worked, the centre diff lock worked.

No Fluid leaks.

Reservoir full.

Tested the brake fluid cap, OK.

Since the fault occurred on the ramp, i ruled out the wheel speed sensors. From reading Adians summary it is clear that the sensors send no signal at rest, but go through a check at 7kms/hr

Reading wiring diagrams, and all the threads i could.

The pressure switch can trigger the fault, and the wiring is directly to the central informer and dash, bypassing the ABS module.

With pump working, pulled the harness off the green anodized switch---pump stops.
This indicates that the relay is fine.R34

According to Adrian Streathers book, the R34 provides over and under voltage protection.
I jumped the pins in the relay base 30 and 87, confirmed that the ABS solenoids open. I never had an abs light, only the PDAS and Brake light. No real way to confirm that the relay was otherwise compromised.

I drove the car carefully.
Brakes good, ABS kicks in OK. Can lock the centre diff and shutter around the driveway OK.

So
Given there was no leak of brake fluid
Cap ok
I narrowed it down to the
a) Pump Pressure switch
b) R34 relay
c) Accumulator

Bleed the system, starting at the Accumulator, the locks, etc.

Pump time cold overnight, 40sec

I ordered the pressure switch and relay

As far as i can tell the pressure switch does 3 things
1- turns on the pump on
2- turns off pump when fully pressurized
3- send controls warning lights if below 102bar

Basically, my system worked as it had prior to the warning lights staying on.

I replaced the switch, and this solved the problem.

Took off the pump and fixed in a vice, not possible to do in the car.

The switch had failed internally, possibly electrical.

cheers
FS
2-

N263ND

Turbohead

I have this if need ed

Goughary

You can connect and disconnect those lines just like brake lines to the calipers.

Torque spec is "snug"...i don't know the correct spec off the top of my head. But it's not high. Just snug it down and you should be good.

I replaced my pump last summer and have had no issues.

Navaros911

Did mine a month ago... well, the mech did it. Paid $312 for the switch. The internal seals were leaking.

I had a lot of spillage in the front, so still dealing with that at the moment.

I drove the car for maybe 20 miles on the road with the brakes not functioning properly. I knew the leak was minor and the accumulator had pressure. Stopped a few times to restart the car, which pressurized the system. So a hairy drive and I used the brakes only 4 times in those 20 miles. I do not advise anybody to do this. I got caught out at the wrong time in the wrong place and didn't have a choice.

PhilMorrison

I'm in the process of doing this job at the moment, and currently banging my head on my desk!

After a serious leak that dumped about a litre of brake fluid into the car, I've replaced the pressure switch and bled the system multiple times. I can't get a brake pedal and the pump makes a lot of noise. I'm wondering if the pump itself is faulty.

It's an 1992 964 Turbo, so doesn't have anywhere near as many bleed points as some cars. Just the 8 caliper nipples on my Brembos & the accumulator (bomb)

I've pressure bled at 20psi, I believe I've followed the proceedure exactly as everyone else does, and in addition I've tried with the ignition on (pump running) and off. I've tried accumulator first, and calipers first. and I've tried traditional on / off manual bleed. I've now circulated probably 20 litres of brake fluid, and can't get any more air to come out of anything, but the pump still makes a lot of noise like it's aerated, and the brake pedal goes to the floor.

Any advice would be very much appreciated. Am I missing something specific to the Turbo model?

Any advice would be much appreciated.

N263ND

Phil,
A few points to consider:
- In addition to the bleed points you mentioned, the non turbo cars also have the dual solenoid valve block in the trunk (1 bleed point), as well as the clutch slave and the 2 locks of the PDAS. These latter 3 bleed points likely would not explain what you described, but the dual solenoid valve block could be a player, if it exits in the turbo.
- If you removed the pump to replace the switch, the return line from the brake fluid reservoir is a frequent source of leaks after reassembly. Many times, it is not pushed all the way in and locked properly. And, in my case, there was only one o-ring, though there should have been 2 (I added the missing one before reassembly).
- Other common points of (air and/or fluid) leaks are the connection between the switch and the body of the pump, as well as the connection between the pressure line and the pump (it has to go very, very tight), but I'm assuming that you've checked those.
- I am also assuming that you plugged the reservoir overflow line during the bleed, unless your car has the upgraded version of the reservoir cap that precludes the need for that.

If none of that helps you, there is a place in California that refurbishes these pumps, which also performs tests on these pumps to rule out a pump problem - www.invasionautoproducts.com .

Good luck!

JasonAndreas

Did you follow the instructions in the turbo workshop manuals?

hunterpeaks

Phil- have you eliminated the master cylinder from the equation? I finally just decided that it had to be the master cylinder by process of elimination. I took it out (careful with those plastic lines coming out of the reservoir - heat them up to remove them carefully!) I sent the Master Cylinder off to White Post rebuilders in Virginia and they rebuild it, I put it in and it fixed the problem.

Hunter

PhilMorrison

I got to the bottom of it after another three hours going around in circles bleeding it... The answer? There is no problem! There never was...... When I thought I'd successfully bled the system yesterday, I had.
Basically, I'm unfamiliar with the pedal feel. . having just shipped the car from Japan.

I wasn't aware just how much assistance the power brake pump applies. Yes I can make the pedal go pretty much to the floor while on the 2post ramp, but when I finally decided to actually drive it, the car locks wheels way way way before it gets anywhere near that point, and drives perfectly normally.

Embarrassing, but it's a steep learning curve I guess. I come from modern GT3s, so I've never felt a pedal like it in the 100's of brake bleeds I've done over the years.

Thanks anyway for the assistance guys. I'll take a look at that workshop manual as I've not seen that before.

Goughary

Makes sense the turbo and the C4 have a pretty squishy pedal relative to the brake booster if the C2 and as compared to most modern cars...

Still- when you say "all the way to the floor"...that's on the extreme squishy side.

Navaros911

All the way to the floor must mean you really stand on it... very hard. If it is easy something doesn't seem right. I'm 200 pounds at 6-3 and find it hard to do.

Also, after 10 brake pedal pumps with car not running it should become almost stiff.

I prefer the feel of the 964 C4 brakes over anything else I've driven (993 feels the same though) and for sure, the ABS will kick in well before the pedal is on the floor.

PhilMorrison

I'm guessing that the big Brembos probably account for the extra travel.