I just got my 1986 XJS back on the road today and couldn’t be more excited. However I noticed that after some time driving the brake pedal will become spongy and need to be pressed all the way to the floor in order to stop the car. There are no leaks and no broken lines anywhere and I am not losing brake fluid. All of this confirmed by my mechanic today who I had check it over before I began driving it. Now I’m wondering what is causing the intermittent loss in braking ability. If I stop the car and wait for about half an hour the brakes are back in normal working condition when I begin driving again. Not sure what could be causing this for certain and any help is appreciated.

 

The cause is most prbably the master cylinder rubbers starting to fail as they get warmer under the bonnet. You should replace them aas a matter of urgency.

 

I agree with Greg. I was having the same problem with my 88 XJS. When cold, the brake pedal would be perfect, then when the underhood temperature increased after about 20-30 minutes of driving, the brake pedal would go to the floor. I was told that it was the MC failing due to heat. The XJS MC was hard to find in the US. Fortunately, I was able to purchase a new OEM brake master cylinder from Paul's Jaguar. Problem resolved.

 

Rockauto have them
https://www.rockauto.com/en/catalog/...+cylinder,1836

https://www.rockauto.com/en/moreinfo...RmSP3kcgmYwOmb

Keep the plastic connectors from your old one, they are getting hard to find.

 

Rockauto lists the MC on their website but if you try to order they say 'not in stock'. I had tried all of the vendors in the US to no avail. Welsh had a rebuild kit which I ordered but when the kit came it only had 2 seals. I think it should be 4 seals. (I may be wrong). I will disassemble the MC sometime in the future and rebuild it for my personal use.

 

Just to toss out an alternative.....

https://www.applehydraulicsonline.com/

I've used this company many times. Good service. There are others out there who I imagine are equally good.

I'd be tempted to buy a used M/C, send it out for sleeving and overhaul, and keep it on the shelf for future use.

Cheers
DD

 

Great idea Doug. Also if you do a search on the part number, there are plenty out there, certainly even in the UK LHD ones are available, as well as rebuild kits.

 

So you say there’s a time when the brakes aren’t spongy? Try mashing the pedal to the floor then and see what happens. and once it starts to feel spongy, just pretend you’re doing a panic stop mash it again- really push the pedal hard to the floor.

See if it has initially has a hard pedal or or soft. See if it’s initially hard, and then it slowly Creeps to the floor or if it holds for a while and then rushes to the floor because each one of those is a different symptom of a specific type of brake problem.

you could be a master cylinder, or it could be a rubber line that’s weak or could be a vacuum booster if you have one

One thing you might try is to take the brake lines off the Mater cylinder and bleed the master cylinder by itself by looping some brake lines from the output back into the reservoir.

A good test for the Master cylinder- you can plug the output ports on the master cylinder push the pedal, and it will tell you real quick if it’s the mastercylinder seals or something else.

Also want to check to see if your brake booster is holding vacuum by putting a vacuum gauge on it if you can. sometimes the check valve goes bad and that can cause your pedal to go to the floor. You can pull a vacuum on the brake booster. See if it holds over time.

 

Turns out this wasn’t the case, at least not yet, there was a vacuum hose that was severed years ago by a mouse, I have since repaired the hose back together, and had no brake issues since. But now I have a hissing sound behind the dash which I can only assume is another busted vacuum hose in the hvac system. I hope I don’t have to remove the whole dash but it might just be the case.

 

Normally, no vac booster would give a very hard pedal and it would be impossibe to push the pedal to the floor; so i still suspect the master cylinder is on the way out.

 

and that’s what I initially thought too until I had a car that had such a low vacuum that after several applications of the brakes a couple times at low speed when the vacuum was completely gone, the pedal would slide to the floor

 

Interesting I wonder why?

 

not sure. This was on a power brake ststem in my cuda with a massive hydraulic cam. Only made about 10” of vacuum at idle. If I was at a crawl in traffic id need to be on my toes as the pedal would slip to the floor after several stop n goes. I put a vacuum reservoir on it (still on it) and never had to worry about it again.

 

It is possible that the brake fluid has absorbed moisture which has lowered it's boiling point. When was the last time the fluid was changed? You could try flushing the system and replacing the fluid. It would be a good idea, especially if you are replacing the master,

 

I used a rebuild kit from Moss Motors. It did the job, for only $16.99.

https://mossmotors.com/rtc1129-master-cylinder-seal-kit