Dick;
Thanks for the reply and info. If you want to go to the private message let me know. If not we can continue on like we have been doing.
Joel
I think I understand
I’m very skeptical of the “heat” diagnosis. Solving this problem always seems to involve fiddling with the master and/or slave. I spent quite some time trying to work it out the last time I had the problem…it finally went away when I replaced everything. Helluva way to diagnose a problem.
If the piston is moving well, I count at three forces moving it back: the spring, air pressure on the diaphragm, and the hydraulic “pull” of the retracting master piston. So plenty of redundancy. But the qualification is, it has to move well.
1 Like
Yes that does seem to be the one most common report, cleaning and lubing the small piston solves the problem. But why does it get sticky so easily? I have wondered if it gets sticky based on the brand of brake fluid. Could be possible, but the dirt on the piston seems to be on the dry side of the seals, not the wet side. So probably not. Type of lube used? Typically only brake fluid is needed on brake components. Many use red rubber grease, works, but just for awhile. Some say original cores work better? Could it be the finish of the bore? There is a report above of polishing the bore helped. Maybe more need to try polishing?
Tom
Still, I wonder then what the purpose of the brake servo heat shield (C26500 I believe) was for. Often missing (including my own 4.2 OTS) and painful to install which may be why so many are missing.
Jaguar must have encountered some issue with heat affecting the servo, though what that effect was I have not seen explained.
Though I don’t have a brake anomaly, I plan to fit that missing shield unless it turns out it really serves no purpose and Jaguar was mistaken in fitting it. Perhaps prolonged exposure to heat eventually triggers whatever problem Jaguar was trying to avoid.
Dave
I actually replaced it with a large sheet of plated steel, lined with fiberglass. I did this to protect the A/C lines, which run dangerously close to the exhaust, IMHO. The stock lower shield provided scant protection. Even with this large heat shield, I had the familiar locked brake problem. My feeling is that even if heat makes the problem appear, the root cause is always something in the master or slave.
John,
I struggled with a similar problem for a long time. Brakes were OK with everything cold but as the engine bay warmed up the brakes would hang up worse and worse until the car was undrivable. Unhooking the vac supply would free the brakes up but not a good way to drive! I cleaned, lubed, replaced the reaction valve, replaced the vac lines, put a stiffer spring in, and finally replaced the master (twice). No luck. Several seasons lost! Out of frustration I replaced the servo and the problem disappeared and has not come back.
In hindsight I think I should have put a delta pressure gauge across the servo vac connections. I think that would clearly show if there is atmospheric pressure bleeding into the servo or not. If the dP across the diaphragm is 0 with the no pedal pressure and the brakes are still sticking it would seem the servo is hanging up. It still doesn’t make sense to me completely even now but an additional bit of info like dP across the diaphragm might be helpful it since would include/exclude the reaction valve.
Pete
1 Like
Same thing when I did my last deep dive. Except that instead of replacing the servo, I rebuilt it. No problem found. When I reassembled, I applied a light coat of silicone grease to the rim of the diaphragm, to help seal it. It worked fine when reassembled. I would try it again without the grease, but we all know what a PITA it is to R&R. So next time, I’ll grease it up again.
1 Like
When I replaced the diaphragm, I used brake grease simply to allow me to rotate the cover to assemble without possibly bunching or moving the seal. The grease allowed it to slide without disturbing the seating. I’d never even try to assemble dry.
Pulled mine apart today …. Here is what I found
The piston required my wife to push very hard to get it to move and even though I was aware what would happen it still came with a very big rush and I still had to cleanup despite rags etc
The piston itself was a little discoloured and was a little rough …… I cleaned it up using 3000 wet and dry to have it smooth as a baby’s $&7
Cleans the cylinder with ear buds until no more crud
Reassemble with red rubber grease and popped it all back in …… time taken 15-20 min
Brakes now non sticky again!
2 Likes
Let us know for how long.
I never needed a hard push, in fact, just used my hand.
1 Like
No this was hard John ……hopefully never need to do again 
Are we talking about the car or last night? 
2 Likes
plus 20
Maybe the issue isn’t so much where the piston ends up after it retreats (the last little bit) but the initial force to move the piston when letting off the brake and a stronger spring might help get the piston moving? Also, when figuring how strong a spring is too strong, wouldn’t the hydraulic pressure on the back of the piston overcome almost any somewhat stronger spring one would try?
Is the mandrill you used in the Shofu gold polishing kit? Which is the “correct” mandrill?
Hi John, the mandrill I used is not the standard size, it is much smaller and comes with the dremel kit and not the Shofu kit. You will probably need to decrease the od of the Brown Shofu wheel with a file as the dremel is spinning. Then switch to the Green to get a smoother surface. The Light Green is ultra smooth and isn`t necessary.
Allen