I recently dropped the IRS on my (55k mile) S3 to replace the driveshaft U-joints. All done and connected the brake line and had trouble bleeding the brakes (shocking!). The pedal was ‘dead’ and i just couldn’t get any firmness to it. Previously they had been working well.
At some point it was determined that the master cylinder/reaction valve was maybe gone. I had purchased one a few years ago so replaced this with this ‘new’ unit plus given the extent of the job, decided to replace the servo diaphram/slave cylinder as well.
Now after repeated bleeding, my brakes are firm, but they feel too hard. In other words - they sometimes feel rock hard while the car does not stop quickly or like it did normally before all this work.
I checked that the manifold/tank check valve was installed correctly and working (air thru the valve in one direction) and when the car is running and you step on the brakes no increase in engine rpm. Also, with foot on brake and you start the car, the pedal does goes down a wee bit. That would suggest the new main booster is working.
I checked and rechecked that I have all the vacum lines hooked up according to the service manual. I understand that the pwda unit is an indicator only and not a proportioning valve. I have no warning light on… If I’m wrong with that could it be that maybe I’m only getting rear brakes? In one of my brake bleeding sessions BEFORE I replaced everything else, I tried resetting the pwda, but didn’t seem to get anything happening. I’ve done this quite successfully in years past on my TR6 and it was a simple procedure. Not so on the Jag.
I’m about out of ideas as to what could be wrong. Any ideas?
A hard to press pedal sounds like no vacuum assist.
I’d plug the vacuum line at the manifold and leave the remainder of the pipe open to the air. That’d mean the whole “assist” side of the brakes will be at atmospheric pressure on both sides of the booster and you’d have no vacuum assist. From your description, it sounds as though there’d be no difference to braking from where you are at now.
If that’s the case then I’d troubleshoot the level of vacuum on the front half of the booster.
kind regards
Marek
The challenge for me where I live is I’m about the only S3 on the raod - no one else to look/listen to when it comes to a similar car… I had a friend who’s a retired mechanic and British car expert come over to have a look at things. It appears that the (new) vacuum booster is working. The pedal seemed to depress when held and the engine started. Another post elsewhere suggested that if the PDWA switch was not centered then it could lead to a pressure buildup because the brake fluid can’t be compressed and therefore a hard pedal. So we proceeded to try to center the switch.
After a few tries, we could not get the switch centered. I had this switch out of the car when I was installing the other components, but didn’t do anything to it other than clean the outside (was working before so…). If the switch is jammed, could it be relieved by tapping with a hammer (the old SU pimp starter) or by “cracking” the bolts on each end (and catching any fluid in a rag…)? Or does it need to be rebuilt or replaced?
I’m not sure if this switch is the culprit or not. When we were resetting the PDWA switch the fluid coming out of front right caliper was strong and clear (of bubbles). The left rear (which is an aftermarket remote bleeder and had been working fine but now is “weeping”) seemed to belch fluid out when initially cracked but fluid didn’t seem to be coming after that. Maybe this is a bleeding problem?
It’s the same principle as any other brake booster system.
You need to be systematic. I have suggested removing the vacuum brake booster out of the equation with a test. You have not responded as to how that went.
FWIW, if you want to, you can remove the pdwa from the equation by bending the brake lines around straight to the booster. It’ll reach if you swap the the front and rear connections. If that worries you, that’s how the s2 is configured.
kind regards
Marek
Thanks again for continuing on this with me. I did a few tests based on this and another forum and here’s where I’m at:
- I disconnected the line at engine to booster so tank and booster were presumably at atmospheric pressure and with engine running no discernable difference in pedal feel.
- I did a test with everything connected, whereby I pushed the pedal and heard a ‘hiss’ to bottom then it stopped and then I released pedal and it hissed briefly until pedal hit top then hissing stopped.
- I pressed on brake pedal a couple of times (you don’t get much movement by the way - still feels ‘hard’) then started the engine. The pedal did ‘sink’ a wee bit but not a whole lot. I will check this against a couple of my other cars. I recall that when one did this, it released more (felt like more of a release).
- I also pulled the vacuum line from engine off at tank, ran engine and checked that there was vacuum on the line (check valve not backwards!). I don’t have a guage to see what it’s pulling, but am in process of borrowing one so will check that too I guess. Can’t see why it would be different - nothing up there was replaced.
I haven’t done any swapping of brake lines as of yet. Could you pull the guts out of the pdwa from one end and accomplish the same thing? Just spitballing here…!
Your first bullet point suggests you have no vacuum assist. Once you get your gauge, I’d check what you have by way of vacuum reaching the front of the booster when the brake is applied. The manual has a detailed explanation of where the vacuum comes from and where it goes when the pedal is pressed and released.
The pdwa has a small piston inside it which separates one brake circuit from the other, so removing that will link all four brake calipers together. That’s a terrible idea safety wise and no one will recommend total brake failure as the only possible failure mode.
kind regards
Marek
Ok I finally was able to break free and do some testing. My thanks for hanging in here with me. FYI - re pdwa - I didn’t pull the guts out. I took the switch out and tried to peek inside - can’t see much… There appears to be no leaking past the seals in there (ie no leaking out switch or ends…)
I did borrow a vacuum guage and here’s what I did and observed:
- At intake manifold (there’s a blocked off nipple right beside the brake booster line so I just used it) vacuum read approx 10". Will vary a bit with engine speed.
- at input line at tank (after check valve) read approx 9".
- at outlet side of tank going to front of booster (I disconnected hos at front of booster and used that line) was approx 8".
-with guage in line that goes to front of booster it was 8 - 9" (engine running no brake application). - when brake applied guage seemed to raise an 1" ( a bit hard to see from where I was but it did jump).
Note when engine shut off in above configuration vacuum held.
Does any of this help or make sense?
Bob
When you apply the brake, you should maintain vacuum at the front of the booster. The vacuum assist happens because applying the brake pedal will vent the back of the booster to atmospheric pressure and the pressure difference is the assist. Releasing the pedal reseals the back of the booster from the outside air pressure and rejoins it to vacuum. There is now no pressure difference as both sides of the booster are at vacuum.
If you have no assist, then you thus have only two suspects:- a torn vacuum booster diaphragm (so no pressure difference is ever possible), or the check valve on the front of the master and its rubber hoses malfunction to achieve the same. Move the gauge to check the pressure on the back of the booster.
kind regards
Marek
Ok here are the results of the latest tests.
I pulled off the hose at the rear half of the booster and connected in the guage. Note - when I was doing this I noticed that the other end of that hose that connects to the front of the reaction valve was not tight! I thought (hoped) that maybe this was the sourced of all my woes. However, when I fired up the car, there was no vacuum at the rear of the booster! Looking like a faulty (NEW) booster!
Then I thought - well in case I hooked it up wrong somehow(!) I should test the old booster to see what happens. So I connected the front of the old booster directly to the engine manifold line (bypassing the tank etc as it is just plain easier physically - vacuum is vacuum right?) and hooked the guage in as before to back connection and viola got boost. So it would seem the old one is fine.
I am sort of kicking myself for replacing the ervo and slave when it was the master that we thought was blown, but I was using the logic that at 50 years of age, and with the brake system being all apart, why wouldn’t I upgrade to new. What could possibly go wrong!
So does this make sense that the new booster is faulty and my checking of the old verifies that? I can’t tell you how tired I am with the hours (never mind the cost) that I have sunk into this issue. Assuming that it’s the booster at fault, I guess I have a conversation with the supplier… I’d put the old one back in, but is that wise given it’s age and whatever the condition of the hydraulic slave is…
Thanks again for your patience and advice.
Bob