More sticking brakes

My problem appears to have been the flex line going from the body to the union on the rear suspension unit. As others have said on previous messages the outside looked just fine but now that I have removed it is plugged up tight so I assume pressure could get thru when pushing on the pedal but then not release. I am now in the process of replacing the front flex lines too.

David Barnes
68 E-Type FHC

Problem solved. Master cylinder, which I replaced two years ago, appears to have been assembled incorrectly at factory. The order of the two nylon bushes was reversed from what is shown in Jaguar Parts Catalogue. Hence the longer of the two bushes was in near the toe of the piston rather than at the heel where it's supposed to be to prevent cocking.

Thanks to all who responded with suggestions.

If anyone still following this thread- Did some calculations and it appears that based on difference in wheel cylinder diameters, static friction coefficients (rear brakes aren't as hot) and servo return spring stiffness (which acts to relieve some pressure in the rears) the force exerted on the fronts when the problem occurs could be 3x the force on the rears. John mentioned this in earlier email but I had to convince myself the difference could be that much.

A residual pressure of 30psi in sticking master is sufficient to produce a static rolling resistance of 140 ft-lb. Maximum engine torque is 284 ft-lb @ 4000rpm.

So I think probably all four wheels are affected. It's just much more obvious in front.

If only one side is sticking it's not the master or servo. Recommended lube for internal bits is Lockheed Red Rubber Lube. Oil isn't compatible w/ brake fluid.

So the general opinion is the little piston (item 31 in my Haynes book) going from the master cyl into the reaction valve is a good place to start looking ? I flushed the system on my 68. All seemed to work fine in the garage and first couple of miles on my drive today in hot Houston but began sticking on the right side. I have not determined yet if front or rear or both. Now that cooled off again are releasing fine in the garage.

Were working fine a few weeks ago before I changed out the fluid in brake and clutch system. The clutch slave cyl blew the day after I changed the fluid but had a bit of rust and trash in it. Now that I have the clutch back working this was the first day I have been able to get it out for a run.

David Barnes
68 E-Type FHC

Go back to the little piston on the ,aster cylinder, after cleaning it , lube with a little oil......
Try it, it works , but if the car has sat for years, you have a cup spot inside the sleeve...

The flex lines are new. Yes, I cleaned and lubed the little piston. Can't be RV as it does it w/ vacuum disconnected. Puzzle is why if it's the master, as it appears, the rear brakes aren't affected. Will post solution when and If I find.

This is (was - fingers crossed) a long running issue on my 1970 OTS; threads on JagLovers. You say you checked the RV after heating the MC. Did you also check the little hydraulic intermediate piston in the end of the MC? If this sticks, then the RV will be held in a cracked open position.

How old is the brake fluid, if its been in the car 20 years its turning into mud. Danny is on the right page. If you tried all the bleeding and vacuum tricks.
Good luck.

How old is the brake fluid, if its been in the car 20 years its turning into mud. Danny is on the right page. If you tried all the bleeding and vacuum tricks.
Good luck.

Old Mushy rubber brake lines @ the front wheels. They look nice and hard on the outside but sometimes they are bad on the inside. How old are the flexible lines? If they are 10-20 years old they might be the problem - did you try to release the bled nipples @ the calipers to see if it releases the brakes? If it did, it could well be the flexible lines.

disclaimer... disclaimer - I am no mechanic, but I did spend a lot of time and money on one of my other cars' brakes the same issue to find it was a $20 rubber line! Replaced them all!

I was able to reproduce the problem by heating the master w/ a heat gun. So I disassembled the reaction valve and found some corrosion at the outer end. Cleaned and lubricated w/ Lockheed Red. Reassembled, heated again and it still does it.

I tried venting the forward side of the servo can to atmosphere and that did not relieve it, so it can't be the RV. Only cracking the bleed screw relieves it. To me this indicates the master piston is not releasing back in the bore when hot. Can you guys think of any other possible explanation?

John,

I began last year by installing a vacuum gage in line between servo and reaction valve. This showed vacuum restored to initial value within a second of pedal release- which does not necessarily mean pressure on both sides of diaphragm is equal. Then I thought maybe the diaphragm piston was sticking so I replaced the servo.

Not long before my car made, Jag reversed the plumbing to the servo outlet ports keeping same servo and master. Wouldn't that suggest that the pressure output from both ports under normal operation is pretty near equal?

OK. Hot today so it's sticking. Jacked the front and cracking either bleed screw releases both wheels.

What's weird is the rears grip and release normally. How is this possible when the rear brake tap off the servo is forward of the front line? Why does pressure remaining in front not continue to press down the intermediate piston and apply pressure to the rear? I've already tried removing front lines, snaked wire thru and blown thru compressed air. I'm stumped!

I installed gage checked vacuum release. It's normal.

Yes I can get it to do it in the driveway when hot. That's how determined only the fronts do it. Never thought to unscrew the bleed screw. Excellent idea. Thnx.

Hi George,

If there is still hydraulic pressure when your foot is off the brake then the problem is somewhere in the hydraulic system. If there is no pressure behind the brake piston pushing it but it is still pressed against the rotor then the piston is being held there mechanically.

Hi Joseph,

Can you make it happen while you're standing still in the garage? If so I would jack up the front end, remove the wheels and have someone press and release the brakes while you watch the calipers. While they are not releasing try unscrewing the bleeder nipple and see if there is hydraulic pressure. If so then you have a hydraulic problem in the booster, master, etc. If not then it is a mechanical problem in the calipers. Whichever result occurs you have reduced the places to look for the problem.

Michael,

I'm no expert, but I would have thought it would be just the other way around - if you have hydraulic pressure at the caliper, then the problem is mechanical. If you don't, then it's in the hydraulics.

What am I misunderstanding?

Thanks.

Hi Joseph,

Can you make it happen while you're standing still in the garage? If so I would jack up the front end, remove the wheels and have someone press and release the brakes while you watch the calipers. While they are not releasing try unscrewing the bleeder nipple and see if there is hydraulic pressure. If so then you have a hydraulic problem in the booster, master, etc. If not then it is a mechanical problem in the calipers. Whichever result occurs you have reduced the places to look for the problem.

Sorry, should say 66 REAR brake outlet is forward on servo.