Unreliable brakes


`I 1987xjsc 50,000 miles lost brakes in traffic after 30 min drive very hot day Florida . Got home with hand brake , no leaks , once cooled have brakes again . Any thoughts? Have had lots of older cars , but never this peculiar problem .

Brake fluid boiling.? Change it.

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Have you ever flushed and replaced the brake fluid? With a non-sealed reservoir (if it is original) your brake fluid can absorb water. If the system gets hot enough the water will boil and pockets of vapor can form. Then it’s goodbye brakes 'til everything cools down.
New brake fluid is the next step, IMHO!

I’ll third the boiling fluid.

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Changed all rotors pads and fluids less than two years ago , have only put 1600 miles on her !

Get a cheap fluid tester and check how much water is in the BF
Here’s a couple of examples

Thanks I’ll give that a shot .

Boiling fluid shouldn’t have happened unless you were using the brakes hard. How did you “lose brakes”? Pedal went to floor? Offhand I’m thinking a rebuild of the master cylinder may be in order.

If master cylinder was the culprit , would it return to working status once it cooled down ? I don’t mind replacing it if that possible . Just a casual drive when they disappeared, but was a very hot day .

Yes, at some circumstances, if the seals are worn - even thermal expansion of the metal will have effect.

However and with your mileage - it looks like typical symptoms of air pocket in brake master cylinder. It will be worst to remove if you have ABS. If non-ABS system - just raise the front of the car as much as you can and do some pumping (engine on, then then with engine off).

With ABS you may need to cycle solenoids (extremely rare) manually which is not difficult, rather time consuming.

With fluid so new I would suspect a mechanical cause for the likely boiling of the fluid. I had a new-to-me XJ with a similar issue. Turned out to be a hand brake retractor spring came out of it’s hole and was pushing one hand brake pad against the rotor at all times. Not enough that you could feel it while driving but enough to heat up the system until the fluid boiled and then of-course, little to no brakes until it cooled. See if you can you push it on a level surface in neutral. Certainly not definitive but a quick way to test if something is obviously dragging.

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Will give that a try this week end . Thanks for the input .

Do not think have ABS it’s 1987 , have seen no evidence of it . I will try pumping it as you say though . Thanks for info .

Still would like a description of “loses brakes”. Does the pedal go to the floor?

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Yes it did . I made it home with hand brake and low gears . It seemed as though as the jag cooled down , I’d say within 45 mins the pedal almost came back all the way . This morning I checked it once again and it was all there . Didn’t have time to drive it though . Thing s I’ve driven it in much longer excursions and never had problems.

See, the thing is, an '87 should have non-ABS vacuum boosted brakes. It should be simply not possible for that pedal to go to the floor even if the fluid is boiling. There are two hydraulic circuits, and if one utterly fails – a hose breaks completely in two – the pedal should only go halfway to the floor, at which point you would still have brakes at one end of the car. I have had that happen, I know what it feels like. That’s why I’m trying to get a better description of the failure here. In general, the only way it should be possible to have a total brake failure on this car is if there’s already been a failure of one circuit and it’s being ignored and driven until the second circuit fails.

So to eliminate that possibility , I’ll order a new master cylinder and install it . See what happens . I just returned to the house and took jag to coffee shop and back no issues, but only a four mile trip was happy but paranoid . I have never thought that the car had exceptional braking.

An infrared temp sensor is cheaper than a new master cylinder, and would quickly tell you if you had a problem that was raising the brake fluid temperature at one corner.

One corner still shouldn’t result in total brake failure. That’s the problem. Nobody seems intent on figuring out what’s going on here.

It may feel like you’ve lost your brakes entirely, however one circuit is probably there. Unless you’ve lost front one due to master cylinder failure (again, highly impossible with 50k on the clock), with rear brakes seized (highly possible) as suggested by Kirbert. Worn master cylinder will fail in the way indicated - however with ABS kicking in (pressure goes higher than usual, then it returns to normal). Just bleed the entire system, inspect rear callipers, don’t touch master cylinder unless you know what you’re doing…
The air pocket can stay in your master for years. Then you need just a decent downhill or few storey car park to “activate it”. That’s why “bench bleeding” was invented…