86 Jaguar XJS V-12 convertible project, Solid brake pedal when engine is off, to floor when running

Getting a little further each time on my project. Went for a short drive up my driveway since it runs and drives and I needed to. The problem I encountered was with the brakes. Peddle goes to the floor but I have brake enough to stop me just on my driveway so no road trips yet. Pressure bled the brakes in the proper order. When the car is not running I have a solid peddle at an 1" or so down. When the car is running is when the peddle goes right to the floor. Not brake fade but straight to the floor. As usual thanks in advance

Simple matter of boost with engine running. N oe when not. Pedal is too low. Air???

If no leaks and no air in system, bad master cylinder.

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Thanks for the information. I have a vacuum gauge mounted on the engine still for trouble shooting and I noticed that the motor doesn’t produce any vacuum at idle until it warms up. With vacuum the brake pedal does improve so I will bleed them differently. If I rev the engine the idle speed will increase to 1100 or so and holds, then I have vacuum of 9in. Then the engine starts to surge and the idle speed increases more, with more frequent surging. I’m making progress all the time with one thing or another. The master cylinder has been rebuilt recently and I know that is the first place anyone would look first but I’m confident it’s correct. I will try to correct the vacuum and surging problem first and I’ll see how that affects the brake problem. Any information pointing me where to look will be appreciated. Thanks Rod

when you step on the pedal when the engine is running, does it effect the engine RPM?
In 86 there isn’t too much to the braking system to diagnose.

  1. master cyl A soft pedal is usually the master cylinder. BUT it could also be calipers that have given up too. See #2
    Also, on (rare hair raising) occasions, coming off the motorway step on the brakes pedal goes to the floor car doesn’t slow down, A huge plume of white smoke out the back engine stumbles briefly, try to pump the pedal up to no avail.
    Diagnosis - Master cylinder shot its wad out the back into the brake booster… the engine sucked all the fluid into the intake and turned into smoke.
    Actually, I’ve experienced this twice. Once in the XJS and in the Series II XJ6 it will get your attention. when a master cyl fails - providing there’s still fluid in the reservoir, you have to tap dance on it to keep it up. It usually goes to the floor almost immediately.

  2. Calipers - look to see if their pissing brake fluid out on the ground or on the front wheels. The calipers, pads, wheels, rotors shouldn’t be wet looking. In most instances, providing you’ve got brake fluid in the reservoir, you can pump the brakes and bring the pedal up, and bring the car to a stop, the pedal will go to the floor slowly - unless you’re standing on the brakes for an emergency stop - then it will go right to the floor.

  3. brake hoses - Same as item 2 above

  4. vacuum driven brake booster - these are a bit harder to diagnose. If it’s functioning, The first press of the pedal with the engine off, should drop the pedal down about an inch or two from the floor, then the subsequent pumps should generate a firm pedal, and should gradually raise away from the floor with each pump. then should be solid. then when you turn the engine on, the pedal should drop a bit, but then be solid again…

5 Check valve which allows sucking, and no blowing. the bad blowing part will cause the RPM’s to shoot up when you press on the brake. It could be the booster or the check valve.
The only way to truly tell is taking the valve off and blow in it, then try sucking or vise versa, depending on which end you’ve got in your mouth. Bottom line, air should pass through this valve only one way from each of the two ends.

  1. a myriad of rubber vacuum lines and maybe metal piping thrown into the mix too that connects this booster to the check valve to the intake manifold. with a can of carb cleaner or starting fluid; on a cool engine (You don’t want this to flash off) squirt each joint and listen to see if the RPM’s go up.

I hope his helps. If you can’t pump the brakes up, my money is on the Master cyl as gone into retirement.


!. Any engine that runs creates vacuum. it is where you measure that is zero. Port or manifold.

  1. If that is 9, that is sure weak. possible cause of little if any boost at the pedal.


Thanks for all the input. Fortunately it all came down to air in the line. My son and I bled it the conventional way this time (pump it, stop, peddle to the floor, yea, ok pump again, hold it). We used a pressure tester the first time and we found out we didnt operate it correctly. So the brakes are all good now. Im moving on to my next problems now. No brake lights and a surging motor. New topic coming up. Thanks for the information.