Trouble shooting my brake master cylinder

[gogo34]

I finally tired of having to pump my brakes twice to get a solid brake pedal so I took it to a brake shop and asked them to professionally bleed the brakes for me. They were able to identify a couple really small fluid leaks but we're never able to get a firm pedal either. When I went to pick it up they we're convinced I needed a new Master Cylinder. The master cylinder is only a year old and has never really given me a firm pedal so I tend to agree that it could be the culprit. When I pump it twice I get a good pedal and it stays firm so I don't suspect there's a fluid leak anywhere.

How do I know if it's just the MC? Can I bench test it somehow? I have the entire brake pedal tower removed but, before I go order another MC, I'd like to make sure it's really the problem.

Thanks,

Gogo34

[SafeAirOne]

Which vehicle? Which MC? Are the snail cams properly adjusted?

BTW...if, in 2 pumps, you are getting a firm pedal and the pressure is holding, I doubt it's the MC, personally.

[mearstrae]

I agree with Mark, sounds like the lining needs adjusted because of too much travel.

'95 R.R.C. Lwb (Gone...)
'76 Series III Hybrid 109
'70 Rover 3500S

[gogo34]

It's a 66 IIa with the CV type Master Cylinder. I've been putting the MC through its paces now that it's out and when I plug the "out" port and move the peddle I get a pretty good pressure build up. I'll try bench bleeding it and see if I can get it to work.

[SafeAirOne]

It's a 66 IIa

What size?

Oh, and I think the entire MC is a red herring. I suspect your troubles lie elsewhere.

[gogo34]

It's an 88. The snail cams appear to be correct. They're pretty worn but, look to be doing their job.

[SafeAirOne]

Well, if it were me, I'd evaluate the piston rod travel on the MC to ensure it was correct and that I was displacing the correct amount of fluid with each brake press.

If that looked reasonable, I'd then have a look at the drums and shoes for wear, and check that the shoes and hardware were mounted correctly, then I'd probably adjust the cams on all wheels so that the shoes were against the drums, then press the brakes to see if I had a solid pedal on the first push repeatably.

If I did have solid pedal all the time, I'd probably correctly adjust the brakes IAW/the bible. If not, I'd probably just bleed the brakes. and try again.

[gogo34]

Well, I was able to make the pedal MUCH better after fiddling with it. I bench bled the MC; rebuilt the brake shoe set up on both front wheels (PO had installed both return springs incorrectly); readjusted both front snail cams and then reinstalled everything; jacked up the front end (to invert the MC) and rebled the front half of the system. It's much better now. Tomorrow I'll bleed out the back, reinstall the wing I had to remove and take it out and double check my results.

I don't know if it's perfect but, for the first time that I've owned it, I can say the pedal's at least firm on the first press.

[o2batsea]

I don't know if it's perfect but, for the first time that I've owned it, I can say the pedal's at least firm on the first press.

This. If the brakes get spongy do not drive the vehicle. Double pumping is a sure sign that you need to do something to repair the brakes. I don't think that jacking the vehicle makes any difference in the bleeding process tho...
The snail cams should be adjusted so that the wheel turns by hand with some small amount of resistance. I usually would set them so the wheel wouldn't turn, then back off one notch (this with the axle up off the ground). Even with the heavy 109 wagon, and the rudimentary brake system, I could lock up all 4 if I really stood on the pedal.