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A couple of days ago I had to perform a "stall test" on my newly rebuilt torque convertor and BW66 transmission. This, of course, required me to step on the brake pedal with a lot of force. I now have a situation where my power brakes seem to come and go. Sometimes it seems to work and at others it doesn't. Any ideas Team?
Assuming the fluid is regularly changed and is in good condition, I would guess the master cylinder seals are failing. I would change the M/C before buying a booster, but if the problems persist do the booster too.
I agree with Greg. The m/c is more likely to get hurt than the booster.
What can happen is this: with age crud develops in areas of the m/c where the pistons don't normally go. Your hard pushing may have pushed the m/c piston (and thus the seals) into the crud. The crud then fouls the seals.
But perhaps you can be more specific about what you're feeling?
Have installed an OEM new master cylinder I ordered from Paul's Jaguar. Didn't want to risk rebuilding the old one. Brakes too important. As a 1975 it has a remote fluid reservoir and found it didn't come with the two rubber seals for the plastic connectors on the top of the master cylinder. So I had to order a kit from MOSS which I'm awaiting. Also decided not to reuse the two hoses going from the reservoir to the two "L" plastic connectors. Which, of course, calls for special brake fluid hoses, that I also had to order (another delay). And... I also decided to order the special brake bleeders to make this a one-person job (thank you LnR). I'll install them today. The seals and hose won't arrive until next week.
New Master Cylinder Brake Bleeders Brake Line Hose for Remote Reservoir
Turns out the back calipers have a slightly smaller thread hub than the front ones. And I don't know what it is. So the fronts are going to get the M10x1mm Speed Bleeders and the rears are going to get what was already on them. The non- Speed Bleeders.
Update - Installed the NEW master cylinder, remote reservoir hoses, bleed the system, and I still seem NOT to have a booster that is working. I've removed the one-way valve at the intake manifold, and I get plenty of vacuum. I tested the one-way valve, and it is working. I removed the vacuum hose at the booster and have plenty of vacuum there also. The brakes do bring the car to a stop, but not without a lot of pressure. Again, feeling like the booster isn't working. I fear the worst. Anybody have any other ideas? Anyone ever have one rebuilt?
Turns out the back calipers have a slightly smaller thread hub than the front ones. And I don't know what it is. So the fronts are going to get the M10x1mm Speed Bleeders and the rears are going to get what was already on them. The non- Speed Bleeders.
The rears should be 3/8-24 thread.
Not sure about the brake booster - following to learn.
Here's another thing I just noticed as I'm trying to troubleshoot the booster. I warm up the car, and at basically all steady rpms at idle, when I step on the brake the engine drops some idle speed and starts running rough. Like maybe a vacuum leak being created in the booster???? Might this prove it to be a booster leak?
Oddly doesn't show the actual removal and says "....you should be able to....". :-)
I tried it that way many years ago. Once. And it was a battle. But, yes, it was doable. Never again, though.
The Service manual (Series III) calls for removal of the pedal box with booster attached. I learned my lesson. I found this method easier. I suspect the Series II is the same
Here's another thing I just noticed as I'm trying to troubleshoot the booster. I warm up the car, and at basically all steady rpms at idle, when I step on the brake the engine drops some idle speed and starts running rough. Like maybe a vacuum leak being created in the booster???? Might this prove it to be a booster leak?
Does your brake booster hold vacuum?
if the 6th bolt is a stud on your pedal assembly you’ll have to remove the left AC fan (tricky) and the electrical firewall plugs to lift the entire assembly out off the car. BUT maybe someone replaced it with a bolt…check under there and see if you have 5 bolts and a nut or 6 bolts..Too bad you did all the other work already but you might be able to leave master cylinder connected to brake lines. check out this link.
You probably will remove the booster. When you get to the fan removal process (2 bolts) apply a vacuum to the fan diaphragm it will close the top flap and open the bottom one. The key is to keep them in that position it cannot creep otherwise you can’t remove the fan. Then you have to wiggle it up at some angle and then down and all of a sudden it will start to come down. Do NOT force the process but spend a good 10-20 minutes figuring out the puzzle to get it loose. Thru the (removed) fresh air vent (bottom of windshield outside) you can see that flap and it hooks on edge of it’s opening. The breaker panel comes off easy loosen the 2 nuts on top of it and remove the 2 nuts at the bottom. It’s a slotted panel so it will slide down. Leave everything connected and be careful not to force or pull on the wires
back to the fan.. as it comes down you will see the power supply cord. It’s a dual plug and unplug it. The tape around the duct will come off and you won’t be able to reinstall the tape around the duct I think it’s done before the steering and dash and dash top were installed…. So you will have a bit of a fan output air leak once reinstalled…
I took this opportunity to clean all the prongs in each plug on both sides and the prongs on the firewall plug. Cleaned it all also.
once the booster is out there is rust and dirt under. I cleaned that as well. I also removed that small vacuum tank in the fender (only time you can do it) and clean (treat and paint) that cavity and remove that foam in there if still there.
(later on do the same on the passenger side..battery tray..windshield wiper motor..firewall plugs. Everything is probably dirty and that foam retains moisture and falls apart).
someone was able to remove the brake pedal assembly without removing the fan. And that depends on that stud. I think he was able to remove it then he replaced it with a bolt. It’s a fine thread and not the coarse thread as the other 5 bolts!
to reinstall the fan (if you removed it) apply a vacuum again and wiggle it up It was easier to reinstall that to remove it.
work in a comfortable environment because if you are uncomfortable and try to rush you will break something. You actually get to do a lot of other things once you remove that booster, namely cleaning under it ect…
check first though to make sure the booster holds vaccum.(or not) Oh when you use your vacuum pump go to 10in of vaccum (like in the video). BUT SOME vaccum pumps leak. Check the vacuum pump on your wet finger or what ever and see if it stays at 10. Pipe of course well connected ect.. my pump from harbor freight leaked unless I kept it in the triggered position..thinking my booster was bad.
Last edited by Jeffkrell; Feb 22, 2025 at 10:27 AM.
Jeff - Thank you for the detailed explanation. I was, however, able to remove the booster by just loosening the bolts going into the Pedal holder and wiggling it out like in the video. So I didn't have to mess with fan housing. Will clean up as you suggested. Applying vacuum with my MightMyte vacuum gauge I could get ANY vacuum to happen. So I'm pretty confident it's the booster diaphragm. Am research getting it rebuilt or buying rebuilt. I see Cardone PN 53-5950 at $162 as a reasonable solution. I'll keep you all posted.
Perfect. I found one at a junk yard off an xjs and it tested good came with brake fluid tank and entire pedal assembly 50.00
i HAD to remove mine to clean that rust and also the rust behind the heat shield…. It’s fun to clean up things correctly the first time so you don’t have to do it again
Getting the power brake booster out was not as easy as that video made it look. I got it out without having to take off the brake pedal assembly. But decided to take out the brake pedal assembly anyway. And as "project creep" would have it, I also took out the vacuum storage chamber and the two plugs and jacks. I've now Ospho'd those areas. Will clean it all up and paint it. I was also able to take out plugs 1 & 2. And even was able to take out the one mounted on the firewall which is also a plug and jack from the inside. Experience has taught me that P&J's are notorious for building up corrosion and now is the time to address that. Well worthwhile. More to come....
Power Brake Booster
