Questions and Answers

Jaguar Brakes Q. Hi. I am having trouble with my 1975 series 2 Jaguar XJ6, 4.2's brakes. I have installed all brand new pistons, seals, hoses and pads in all wheels and also the hand brake. The brake booster and master cylinder were supposed to been just replaced by the previous owner.

After flushing the system and putting in the recommended fluid and bleeding (properly, I get a very hard pedal and very poor braking. I suspect that the brake booster is not doing it's job. Although when I pump the pedal a couple of times to expel the vacuum then put light pressure on the brake pedal with my foot and start the engine, I can feel the pedal get pulled down (not greatly).

Could you let me know what vacuum should be at the end of the vacuum pipe just as it enters the booster? On testing mine there with a vacuum gauge I get "0" gauge (minus 10 lbs per square inch). I do not hear any hissing from the booster when the pedal is worked. also would you be able to tell me the adjustment for the push rod, from the booster to the master cylinder?

Best regards and Merry Christmas to all.
Derek

A. You should have full engine vacuum at the booster, about 18" of vacuum. Any less then that you have a vacuum supply problem. Here is the way Jaguar wants you to check the system:

Testing and Inspection:

NOTE: The following tests on the vacuum system should only be performed if the hydraulic system is in satisfactory condition.

1. Raise and support front of vehicle. Be sure one wheel will rotate freely.
2. Start engine and allow vacuum to accumulate.
3. Apply brake pedal several times. Be sure wheel is free to rotate immediately after pedal is released.
4. If brakes bind, booster is faulty.
5. With engine running, apply brake pedal several times and check operation of pedal.
6. If response is sluggish, check condition of vacuum hoses and booster unit air filter.
7. Allow vacuum to build up, then switch off engine and operate brake pedal.
8. Pedal should be vacuum assisted for approximately three applications. Any less indicates a leaking vacuum system or an inoperative check valve.
9. With engine switched off, operate brake pedal several times to evacuate any vacuum left in system.
10. While maintaining light pressure on brake pedal, start engine. If booster unit is operating correctly, pedal will fall under existing pressure.
11. If pedal remains stationary, there is a leak in the vacuum system.
12. With engine running, apply and hold brake pedal. Switch engine OFF . If pedal rises against foot pressure, booster is faulty.

I don't have a specification for the length of the push rod between the booster and master cylinder. I haven't seen one in about 20 years but if I remember correctly it is a zero clearance fit.

Additional Information provided courtesy of ALLDATA