This happens continually with every subsequent application of the brakes. We are going to check all 4 calipers today by compressing the pads into the calipers and looking for problems.
If the internal valve has not released, your brake lines will be pressurized and I doubt you will be able to move the pads. If that is the case, loosen a bleed screw to release the pressure. If it does then you need to adjust the actuator rod (ie. loosen it)
I have removed all 4 wheels while the car is back on the lift. Right now I am trying to remove the pads, and the tab on the top of each pad is blocking the keep plate as shown on L-12 in the manual. I am not getting the bolt through the ākeep plateā because of the tab on the top of the brake pad itself.I am checking to see if I have any seized caliper pistons. This really should not be this hard! Perhaps there is another way to pry the pads back without damaging the pad or the rotors I have plenty of meat left on the pads themselves.
You need to bend the tab a little to extract the keep plate bolt.
Did you go through the booster already?
Did you loosen a bleed nipple to release hydraulic pressure? If the brakes then free up itās a clear indication that itās not a caliper problem. Check the booster.
My wife thought to bend the tab, which we did. Perhaps lucky that we are removing all pads to check for seizure. We spent an hour an a half removing just the right rear pads. I use Kroil (rust penetrate), brake grease on each side of the pad and relentlessly tugged on the hook to no avail. I did loosen the bleed nipple which did not give any assistance. Finally, and perhaps a tip to others, we loosened the four bolts holding the piston retainer to the caliper bracket which pulled the pad away from the rotors.
Yes, Iām sure its the booster or the valve on the booster as noted. I want to follow my local gurus advice to insure I have covered all bases to his satisfaction.
I gently files down the edges of each pad to remove any obstructions and I am going to use a gun bore cleaning wire brush hooked up to my drill to get any rust out of the areas where the pads slide.
I am using copious amounts of spray brake cleaner to keep things sanitary. Its been many years since the pads were removed and they seem in good shape. I have sprayed more Kroil on all moving parts for a second round of fun tomorrow.
Is there actually an air control valve on the servo itself?
G
Iāve come to this late.
As far as I can see the only way that the booster can make your brakes slam on hard is if the poppet valve that allows air into the atmospheric side of the pressure chamber is sticking open. This would allow continued boost up to maximum without additional pedal pressure.
This is obviously quite dangerous, both because even gentle pedal pressure may produce a full braking effort, and also because releasing the pedal may not cause the brakes to come off.
Your booster is faulty and should be replaced.
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If youāre in this far you can rebuild the calipers!
Little money, few hours and often worth it. Extracting the pads was the difficult part
Not sure if we have the patience/skill for a rebuild right now. Itās been one thing after another starting with the starter, the proper a/f mixtures, a misfire due to a faulty secondary wire on #4 and now vacuum problems. I am lubricating all friction surfaces, except for the rotors and pad facings of course. The pistons at this half way point are all moving smoothly. I have used a wire wheel machine to nip just a mm or so off the sides of each pad and cleaned off the surface rust on the back of each pad where it touches the pistons. Lots of meat still left on each pad. Going a bit overboard, I am wire wheel cleaning, priming and painting the pad hold down brackets. We got a case of shipwrights disease. I want to match the powder coated finish on the calipers.
So in pulling a pad it was so rusted in that the back plate broke off and I had to resort to a BFH to knock that puppy loose. Luckily I have a spare pad. This has been such a great bonding experience (wink wink) for me and the beautiful Angela. She is SO mechanical and patient.
G
We had the IRS out and didnāt expect to, cleaned it until the parts came, and it was straightforward and easy.
If you want to look into one, clean off all of the oil, let the brake cleaner dry, pry off the dust boot and pull out the piston. You wouldnāt struggle with it (you have a wire wheel, and skill) and it wonāt take long.
I might have found the culprit of the brake problem. The air check valve on the Reservac is allowing me to blow and suck on each nipple. I could not hear this before as I had other background noise and my hearing is on a down hill slide.This indicates a failure of this valve. I will order one up and if that doesnāt fix the problem a booster is next. I will also order new brake pads as a matter of course.
That doesnāt explain your problem. That valve simply holds vacuum in the tank. These valves do fail regularly and what you get is less boost on repeated brake application.
As Iāve said above I think your problem is likely to be in the booster, and it is a potentially dangerous one.
Itās not likely to be due to a stuck piston either. What you see there is that the car will pull to away from the side of the stuck piston on braking. Your brakes will be less powerful, not more.
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I will be testing on our secluded road at low speeds. This is broken anyhow and I realize by your advice and others that a booster is in the works. Thank you for your support.
This is (I think from the thread) a Mk2, so it doesnāt have an IRS.
It has the Dunlop wheel cylinders front and rear. Unlike the XJ6, the bore of the cylinder rather than the piston is the sealing surface.
So if the cylinder fails the body of the cylinder needs sleeving, which is neither cheap or easy.
With later calipers like the XJ6 itās a simple case of replacing the pistons and seals and youāre good to go.
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I had all 8 re-sleeved in SS 15 years ago for about $900 USD. Iām sure that would be a lot more these days!
More than welcome. Threads on malfunctioning brakes always scare me a little 
If your car has the original booster you may find it hard to get it rebuilt because of non availability of parts.
I faced this problem recently and ended up buying a later model Lockheed booster. This works very well but isnāt a straightforward bolt in replacement. I needed to do a bit of modification to the nacelle that the booster mounts into as the Lockheed booster has a different mounting bolt pattern.
This is one of those updated models which had to have some modifications to install within the wheel well. Brake problems scare the S&*t out of me and the consequences can be deadly. We had the throttle spring fall off on the GTO with an Edelbrock and it went wide open with over 450 HP. I turned the key off only allowing one pump of the brakes and extremely hard steering. I now have a double spring almost fail safe system.
In my experience if theyāve been done in SS they last pretty much forever. In that case itās a simple as just cleaning them and putting in a new seal.
Thatās good. They can be rebuilt although itās often as economic just to buy another.
I had the throttle jam open on my 4.2 E type once; absolutely terrifying.
My local Guru insists I use LMA brake fluid and flush the dot 3 or 4 in the system. Heās picky and grumpy but it never hurts to purge the system. I think Iāll try gravity bleeding first, then the pump, pump, Hold with my beautiful Angela - aka one bad ass mechanic!
I have to take a rest now as I pain meds are wearing off. Ugh 4 knee surgeries, the first two of which were complete failures for total knee replacements and the tendons tore. Thatās a special kind of pain.
Send me a picture of your herd and something scenic about Australia.