Hello All.
I drove my 1988 XJS today to get the exhaust system redone and about 15 minutes into the drive my brake pedal went to the floor. Since I was about 5 minutes from the muffler shop, I nursed the car there with the car barely stopping with the pedal all the way to the floor. We got the car up on the lift and about 3 hours later the exhaust work was done. The shop owner told me to drive the car off the lift and lo and behold the brake pedal was high and the brakes working again as if there never was a problem.
On my way back home after about the same 15 minutes, I lost the pedal again. I got the car home and parked it in my driveway and after about an hour, the brake pedal was back up again and working as it should.
During the restoration, I replaced the 2 front calipers, serviced the the rear and replaced the brake pads. The only part that was not touched was the master cylinder.
I had my neighbor come over and he pumped the pedal while I bled the air out of the system. When driving the car, there is no pulsing of the pedal that would indicate warped rotors.
I have never had a brake issue like this ever, so I have questions. It is somehow related to heat.
When the brake system is cool, the brake acts normally, so I would not know if I fixed anything until I drive the car.
1. Why would I lose the pedal after driving for a while?
Pumping the pedal does not bring the pedal back up.
2. Why would the pedal come back when the system cools down*?
3. Is it that there is still air in the system?
4. Is this a Master Cylinder issue?
5. Anyone had an issue like this before? What was the fix?
Thanks.

 

Classic symptom of master cylinder rubber seal failure. As the temps heat up the seals give and thus all you are doing with the pedal is pushing the seal and the fluid is going past them, rather than down the pipes to the calipers. A new M/C, or re-seal kit if the internal wall of the M/cylinder is still pit and rust free, is the solution. Assuming you do not have anti lock brakes. Rockauto have them.

 

One other possibility is that the fluid has absorbed too much water, lowering the boiling point. When brake fluid boils, the vapor is compressible.

 

OK. Since the only part of the brake system I have not touched is the Master Cylinder, I am going to order one.
My system has the remote brake fluid reservoir with 2 brake lines going to the master with hoses on each end of the lines.
The Master has 2 plastic press fittings where the hose is attached.
Anyone knows the part number for the master cylinder.

 

Sanchez
Make sure you keep the plastic ports that enable the M/C to join to the reservoir! These are NLA and the new M/C will probably NOT come with new ones. These ports just press into a rubber bung, very carefully prise then out from the indent in the rubber.
This is the Rockauto link to what you need, by the looks of the photo you may be lucky with the ports too!
https://www.rockauto.com/en/catalog/...+cylinder,1836

 

Greg: Item Out of stock at RockAuto, Carid, Ebay, SNG, Moss.
There is availability if I needed one for a RHD.
I purchased a kit from Welsh Enterprises but the kit just comes with the 2 seals.
If I fail with the seals. Welsh has one for $487.00 plus shipping. $200.00 Core Charge

 

Sanchez
Try this:
https://www.ebay.com/itm/37417551233...512338&vxp=mtr

Also worth searching on CAC1583, I turned up quite a few.

 

Greg, Thank you for that link. I spoke to Paul at Paul's Jaguar and he had the last one in stock. I was able to purchase the MC from him.
Since I am a past customer and had visited his store in Ft Lauderdale I got the MC for a decent price and no tax.
Should be here on Monday. Paul is 2 hours away from me and if I wanted I could have driven down there and picked it up myself today. With the cost of gasoline being what it is today, I opted for postage.
I will rebuild the MC I have with the seals from Welsh if the MC bore is not pitted and put it up for sale.
Thanks again.

 

Almost 3 years later, I still have the above problem. I replaced the MC with a NOS one but got busy doing other things. I took the car out this morning and the brake was fine for about 10 miles then the brake failed again but this time I still have front brakes, even though almost all the way to the floor.
Since I did not touch the rear brake over the years, I am going to replace the rear calipers. I just have to order the rear brake hose once I have the hose I will drop the rear suspension and replace the calipers. I am too old to try to do it on the car.

 

If you can get the problem to recreate itself, would be worth getting a cheap IR thermometer from eBay and when the pedal goes to the floor, lie down on the pavement and point it at the rear discs; see how hot they are.
Could be a sticking caliper in the rear, boiling the fluid in the calliper/line. Temperature measurement will confirm it 100%