I have a late 1995 XJS, VIN 197471, and shortly after replacing all the pads I lost most of the brake pedal pressure, a classic master cylinder seal failure. So I need a brake master cylinder rebuild kit. But there seem to be none available for the later cars after VIN 179736, I've scoured all the usual suppliers. So the car is sitting unusable for want of a few simple rubbers. Any ideas would be greatly appreciated.

 

You'll likely have to buy a complete master cylinder. Rock Auto appears to carry them. Be careful, as there is another part number change after your VIN.

 

As you probably know, the Teves ABS doesn't use a traditional master cylinder. It uses a very complex actuator unit. The "master cylinder" is merely a chamber at teh front which controls fluid flow to the BAS valve block to support front wheel braking anyway.

I would be extremely surprised if your problem is seals in the actuator unit. It may be so but I do not personally know anyone who has ever replaced the seals in the actuator unit. In my experience 99% of Teves ABS problems are nothing to do with the actuator unit.

When you replaced the pads, you didn't push back the pistons without opening the bleed nipple to allow fluid to escape did you? Otherwise there is a real risk of pushing fluid back to the ABS valve block and causing significant problems.

I'd suggest you start the diagnosis a different way. First of all, discharge the pressure in the system. Turn off the ignition and press the brake pedal firmly 30-40 times. It should then be really hard. If so, turn on the ignition and count how long it takes the pump to run to charge the accumulator. The pump should cut out in less than 60 seconds. Now test the brake pedal.

Let us know how you get on and whether any of the above doesn't go to plan.

Cheers

Paul

 

That vin 197471 is an early 1995. You should have the Teves 3 hydroboost style master cylinder. AFAIK those are specialist rebuild only. Post a pict of you MC for be sure. IF the reservoir is a perfect triangle you have the later brake system. Squarish is early.

 

Vin change is 198335, so it is the complex Teves system.

As mentioned, really think its unlikely to be the actuator.

Cheers

Paul

 

Many thanks to all for the information and advice. It is entirely likely that the mechanic who changed the pads did not release the bleeder before pushing the pistons back, and the fluid in the system would not have been flushed in some time so that would not have helped either. I'll try the test described by Paul, it sounds like I'll need to get the actuator rebuilt.

 

Did the mechanic bleed the system like a standard system? The Teves system is bled much differently.

 

Hi isgrant

Before you go throwing any serious money at the problem, as has been already said its unlikely to be the Actuator as they almost never have been known to go wrong

Instead first get the Brakes Bled as suggested by Brewtech

There is a Special Way of doing this which has been perfected by Paul (ptjs) who can explain what to do, as it also requires the use of the ignition

Though first of all you have to Bleed the Low Pressure Side of the System, or else you'll be wasting your time as the Brakes just won't Bleed, 'ask me how I know or rather don't'

How to Bleed the Low Pressure Side of the Teves Mk 1V System (which is not as scary as it sounds) except this was a very early Post How to Bleed The Low Pressure Side of The Teves Mk 1V Braking System

 

Many thanks to all for the very helpful comments and advice, very educational. As best I can tell, the mechanic who changed the pads simply pushed the pistons back and didn't relieve pressure with the bleeder and didn't bleed it afterwards either. We've been held up by flu here but were finally able to get to Paul's suggested test. After pumping about 25 times the pedal went hard, and when we switched on the ignition the pump ran for almost exactly 60 seconds. Now the pedal seems to be holding so it almost seems as if it's fixed, we had not thought to try pumping it for so long. We'll be able to drive the car in a couple of days and I'll report back on whether the brakes are holding, or perhaps Paul will have further advice. Thanks again.

 

isgrant,

It's good to hear that it seems to be working ok, although the routine I suggested shouldn't have changed anything. I suggested it merely to see if the pump was operating and cutting out within an expected period of time. That SHOULD indicate that the pump, accumulator and pressure valve all SEEM to be functioning ok.

I think you'll really only know when you (carefully!) drive it. If things are less than perfect, do write back and we can suggest the next step of steps for you to consider.

Good luck

Paul

 

Paul - many thanks for your advice so far. Here's the latest from the car. The brake pedal feels normal now when you first start, but the brakes seem weak at normal pedal pressure, then as you increase pressure they suddenly grip really hard. It's as if only two of the pads are acting at first. It doesn't pull to the side with either light or heavy braking. Any thoughts on that?

 

That possibly sounds as if you're getting a bit of pad knock-back on the front through very slightly distorted discs, or else the pads are slightly sticking on the shoulders of the caliper and then releasing? Perhaps start by seeing if the pads move freely in the caliper, (maybe copperease the edges?). If you're going to exercise the pads to get them moving, remember to connect a pipe to the bleed nipple and let any fluid release that way. Maybe also carefully clamp the flexi-hose. Don't let fluid get pushed back to the valve block.

Good luck

Paul

 

Hi isgrant

I'm with Paul (ptjs) on the Sticking Brake Pad Theory

As I've had this happen a couple of times myself, even though the Brake Pads slid in Ok to begin with but when I went to renew them, I had to take the old ones out with a FBH and Chisel, which took me over an hour, so following on from 'Greg's' advice

What I do now is to shave a 'Mil' off the side of the Brake Backing Pad with an Angle Grinder, so that you can put them in or pull them out with just a Finger and Thumb

See Link (with photos) Shaving a 'Mil' off the Brake Backing Pad to prevent them from Sticking in the Calipers