A friend’s car had a new master cylinder and servo put in some years ago and I just found out that since then, he has to pump the pedal once (to build up pressure) before the bakes apply. He has had his mechanic bleed both brake circuits several times but no cure. The servo assist works.
Could this be a bleeding problem only? On my 69, I have never had a problem bleeding the brakes manually. But I have read many forum threads where a power bleeder has been used.
Are there cases that you have experienced where manual bleeding simply cannot not get rid of all the trapped air?
Thanks
Dennis 69 OTS
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
2
This problem can be caused by warped rotors or loose wheel bearings.
At the time of the work, was the brake fluid changed from Dot 3 to silicon by chance? If so there is likely residual old fluid in the system (even with draining and bleeding), and because the two aren’t compatible, it can lead to fade symptoms.
After reading many threads, I think that the new replacement servo is can only be bled by pressure bleeding so I will rig up a bleeder by sacrificing a reservoir cap and handpumping a slight amount of pressure to push the fluid through!
Thanks for everyone’s comments
Dennis
When we have to solve simple tasks like bleeding a simple brake system like these “E’s” by using power bleeder and other “involved” methods I ask myself : how did they do it on the production line while being built? The manual does not reference any special tools that I can recall…
Just been through this - two years, new m/c and booster without making a difference. Turned out was the front pistons - the piston retract mechanism was not working and pulled the pistons back .030 after braking. Doesn’t seem like much but that was 1 -2 extra pumps on pedal. Pistons were stainless steel ones from one of the usual’s - were crap. With a wheel off look down between the rotor and the pad - if the pad doesn’t seem to be in contact with the rotor that may be the problem - or could be a warped rotor but you should feel that when braking…
I agree, 10 - 12 psi is enough to get good flow. Shouldn’t cause any problem for a properly clamped reservoir hose but still a good idea to stuff a towel below them.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
11
I have pointed out the issues with GT Jaguar stainless steel pistons on the XJ-S list, but perhaps worth repeating here: The OD surface of a brake caliper piston, regardless of material, needs to be ground and polished to a mirror-smooth finish. This is because this surface slides against the seals. GT Jaguar’s pistons came with a machined finish. After some use, they’d eat up the seals and the caliper would start leaking. They gave stainless pistons a bad rep.
Stainless pistons are the way to go, but if the ones you get don’t have a polished OD surface you need to give them one before installing. I managed to polish up my set:
Interesting, I was always under the impression that the seal did not really slide on the piston under normal use. As the piston would move in, it would “roll” the seal slightly, then when the hydraulic pressure was relieved, the seal would “unroll” thus slightly pulling the piston back away from the rotor so the pads would not be dragging on the rotor- or at least dragging with no pressure. Thus the piston would only slide on the seal as the pads slowly wore.
Tom
