Brake puzzle

I had also replaced my servo and mc. All was great until the hot weather kicked in. I checked that all my vac hoses were good and tight. I recently fitted the heat shield as my car didn't have one - no change.

I noticed that when it happened on mine, the brake pedal would have slop after a stop. After a few seconds it disappeared. I took apart the mc again and slathered red grease all over the seals.

We have hot weather and I have taken it out for some 20+ mile high speed runs as well as stop go traffic to get it good and hot. At the moment all is good - fingers crossed.

I suspect that the main piston in the mc was sticking - time will tell.

Lots of useful info on Jag lovers, but I've already ruled out all the common reasons. It's not the RV, vacuum hoses, flex lines or secondary servo piston. I think the big plastic diaphragm piston is not releasing fully. Going to fit a servo heat shield and see if that fixes it.

Thanks all for the help and advice.

I am not sure, it has been a while, but I think the pedal was very hard. After several minutes/ hours, the pedal and brakes returned to normal as the pressure leaked off. Cracking a front bleeder also fixed it.

I could imagine the piston returning enough to shut off vacuum to the booster but not enough to fully release the wheels. If you think that seems like arm waving, you are right. But it might be correct.

Bill

This has been a long time issue on my S2. Lot's of good troubleshooting information over on the Jag-lovers forum, where I hope that I finally have the cure. Time will tell.

Bill,

Yet another possibility to consider. Did you notice any play in the pedal when it happened?

Don't you think if master was sticking I'd have seen some indication in my vacuum test?

Whatever it is, there's not enough pressure for brake lights to stay on. I'm thinking I may have to remove the pressure switch and rig up a pressure gage to see what's happening.

Thnx, J

Joseph,

Something else to consider. The master cylinder has a reasonably large pressure release port which is uncovered when the piston retracts. It also has a very small port which, if plugged like mine was, does not allow the piston to move far enough expose the main port. Caused the front breaks to stick, not the rears, I am not sure why. Answer was to CAREFULLY drill out the port which fixed the problem. SNG Barrett has the repair kit if you decide to dismantle the cylinder. Hope that helps.

Bill Braun

Key words- "I hope."

Bled fronts and fluid was dark brown, less than a year old. Pedal seems more firm. There must be crud dissolving off inside of hard lines, which were not replaced. Going to replace tubing and caliper piston seals and hope that does it.

Thnx all.

Eureka! I hope. Drove around the hood and the brakes didn't drag. The car rolled forwards and backwards on very slight slopes, which it didn't before my caliper rebuild. All I need now is a 50 mile or so drive to be sure the problem is cured.
Got my seals and pistons at a Dallas Brit car place. They're available as kits from all the usual Jag parts sellers on the internet. Your closest is SNG Barrett.

William,

That's very interesting. There's one or more distinct release/pops when it lets go, which I thought might be pads unsticking from rotors. It does seem to get progressively worse, which could be as more seals get hot enough to stick, until they all do it. It's the only explanation that seems to fit all symptoms.

Where did you purchase seals that fixed it?

Thnx!

I have read that one common E-Type issue is 'sticky' brakes on cars that are not driven regularly. Mine was out of commission for about 18 months for a partial restoration which could be the cause in my case. I also hear that switching to DOT 5 without changing seals and flushing/cleaning the entire system will make the brake seals swell.

I had a similiar problem with a 66 which gave us a lot of trouble, it turned out that the seal kits rubber was too soft causing the pistons to stick on.

Alan,

That's what's most puzzling- on later cars the front port is between primary and secondary servo pistons, so if primary sticks it should affect the rears too. I replaced servo and didn't fix. Wheel cylinders were sleeved but have original pistons so I'm wondering about retraction springs. But it's inconceivable they would all do it simultaneously. There's no tendency to pull to one side or other when problem occurs. Pedal does not go spongy, so I don't think its water in the fluid.

Going to try Dick's suggestion of cracking the bleed screws. If it's the servo, there will be pressure in the lines. If wheel pistons are sticking, there won't be. I'll let you know if I figure it out.

I have the same problem with my Series II on longer trips. Seems the hotter the brakes get the worse it gets etc etc. It has new master cylinder/reaction valve and S/S hoses. Some say its the brake Servo (plenty of vacuum at the Servo hose connection to the reservoir). It seems to be the single inside front caliper pistons which are sticking (they're all pitted) so I'm replacing all the front pistons and seals as we speak. If that doesn't work then it's a new or rebuilt Servo, which can be an expensive pain. so fingers crossed. Rear brakes seem OK but I guess the Servo setup could affect just the front?

Dick,
I replaced them. There was a lot of crud so maybe some got left behind, I'll try blowing everything out w/ compressed air and refilling w/ fresh fluid. Thnx.

Try opening the bleeder screw on each side and see if the wheel frees up. If so, it is the flex lines. They do close up internally over time.