Getting air in brake system

[bmw8890]

Hi,

I re-sleeve all the brake components except the servo cylinder on the1963 Favia brake system. The brakes work great except after driving the car about 100 miles or so the brake peddle travel increases without any change to the fluid level. To correct the peddle travel I need to bleed air out of the MC thru the servo cylinder front bleed valve using the MC rear plunger and also working the brake peddle. So it is the front brake circuit that is causing the issue and the MC rear plunger resistance never firms up like the front plunger.

Has anyone experience this problem? Is this due the porosity of the MC casting?


Thanks,

Brian

[LanciaFin]

Hello Brian, have you checked that there is no brake fluid in booster? Also, does the main cylinder piston come completely to rest, when the pedal is not pressed? BR, Miika

[bmw8890]

Hi Miika, No fluid in the booster and the booster rod seal is adjacent to the rear brake circuit that is working fine. I checked the MC rod at the peddle and it is making a full return. Thanks, Brian

[FL30]

Brian, the rear circuit of the booster is operating the front brakes (it is a Lancia). I bet the air is coming in exacty at the booster rod sealing. Had that several times and am having it at the moment with my Flavia Vignale. No liquid visible in the booster chamber. Bleed the front brakes and booster circuit only, and everything will be fine for the moment. After that, leave her in the garage for two weeks. Then go for a ride, the problem will be there again. Prove me wrong, always willing to learn!

[bmw8890]

Hubert, Guys thanks for your input. I do remember using the old rod seal because the rebuild kit seal was for a larger rods. Is the rod seal available without buying a complete rebuild kit? Thanks, Brian

[FL30]

As far as I know, you have to buy the complete set. The quality is arguable. I have to replace the seal every four to six years, but have no idea where to get better stuff. I have made a different guide bush for the booster rod, using o-rings instead of the original lip type seal. Will install that for testing when reparing the Vignale.

[bmw8890]

Hubert, Can you send me the seal bore (inside and depth) and rod dimensions so I can search for a replacement seal? Brian

[LanciaFin]

The quality of rubber parts has gone down globally. It may be useful to adapt some more standard seal for the rod, but I doubt that o-rings would do the job. It will be interesting to see if works. I've recently acquired a Flaminia Berlina, which will need a complete brake rebuild, so this issue is waiting over there too. The seal kit I bought from Franz Heilmeier some years ago (it was installed a year ago however) has done its job so far very well in my Flavia, no problems with brakes even after winter break. BR, Miika

[FL30]

I discarded the O-ring idea. Instead we did some homework and "developed" a much more durable part. You can see some pictures here:brake booster seal click pics to enlarge

[LanciaFin]

Very nice! Now the obvious question is, are these available to rest of us, somehow?:)-D Regards, Miika