Brakes work great until yesterday, It is really hot and muggy out while driving. I notice my brake petal needs to be pressed a little more then usual. about 30 mins later the petal sinks even more. about an hour after that i have slim to none brake pressure, and it no longer stops the car it just slows it down. THINKING i checked the fluid, its full. I checked and pumped the brake while in the driveway, nothing no pressure. I check the ground and the hoses, no leaks. I leave it set for about 2hrs come back out to really start invesgiating the problem, and to my surpise i have full brake pressure. When i turn the car on and press the brake pedal i can feel it start to relive the pressure. Someone told me it could be master cyclinder or he told me some sort of vacumm failure. I thought it might be a valve that is going bad letting fluid back in somewhere.

 

Are you using DOT 4 brake fluid? It sounds like the fluid is getting hot, or even boiling while running. Castrol GT-LMA has a high boiling point, but the Jaguar Super DOT 4 from your Jag dealer is the best-500 degree boiling point.

Others may have better ideas on seals, cylinders, etc as you mentioned that may help you out.

 

If the pedal sinks, it's got to be master cylinder.

 

hmm would the master cylinder go in and out? i would assume if it was bad it wouldnt build pressure back up after sitting, is there a wayy to check the master cylinder?

 

It's not common, but can happen. It's probably some sort of internal bypass around the seals. Temp may effect it.

 

lets say the MC is bad, how do i go about replacing it?