Master cylinder

[Atlee]

Just finished rebuilding all four calipers, new brake lines front and rear, and............
Bled rear right caliper, no problem,
bled the left rear and got fluid coming out of the bleeder valve but got no pressure on the pedal even after ten or so go rounds.
I can't see any leaks from any wheel or under the master cylinder.
The reservoir is full. so.........
Is there anything else to look for before replacing the master cylinder?
The beast is a 69 1912 and has been sitting for about five years before I got it.
If I do get into replacing the master cylinder, are there any demons to look out for?

Danke Schon


Ron

[dimps8]

Don't replace the master cylinder just yet. Heres a story which may save you money.(i found out the hard way).$$

I rebuilt my seized rear calipers, bleed all the lines, etc. Still the pedal was soft and could go all the way to the floor. I asked myself what have i missed, must be the master cylinder. I removed the master cylinder, put a kit through it, bleed brakes, furthest away from master cylinder etc, still spongy. Must be a crook bore. I purchased a new 19mm master cylinder $210.00 Aust, installed it, bleed brakes again, then to my horror it was still spongy. Damn! can't be brake lines there ok. Then i thougt what have i missed. Then it occured to me. The front calipers have one bleed per caliper, the back ones have two bleeds. I missed bleeding the inner caliper half on each wheel. Bleed these and hard as a rock!

for sale- one reco 19mm ate master cylinder!

[Atlee]

Nice idea mate, but I had the calipers apart on the bench, and I am sure there was only one bleeding (oops.... Bleeder) valve on it. However I will pull the wheel again and check it.


Good on Ya

Ron

[Green 912]

Did you reinstall the calipers with the bleeders pointing up? If not, you won’t be the first to mount them upside down.

Try bleeding another caliper to see if it will flush.

Were you careful to keep the master reservoir full of fluid while bleeding? If the master gets full of air it can be hard to reprime it. A pressure or vacuum bleeder is a good fix in this case.

The master cylinder might have rusted out inside past where the pedal normally moves and when you pump to bleed the piston seal grinds over the rusty cylinder walls and gets cut up. If this is the case you have a master replacement in you future.

I thought stock “M”s had one each and “S” and “A” had two on the fronts only? Do the early (pre 69) have two?

[Morrie]

My 67 has 2 bleeders on the rear per caliper and 1 per caliper on the front.

Morrie
67 912 Coupe

[Green 912]

Thanks, I checked a set that I pulled from my 69 amd then I rememberd that I put "M" fronts on the back when I installed "S" in the front. Duh...

[r22assessor]

Rebuilt all four calipers and single master cylinder on my 1967 Porsche 912. Reassembled everything and could not sustain pressure - pedal went to floor. I could "pump it up" to the point that the brake lights would go on, but I would lose it every time I fully released the pedal. I plugged the left rear caliper, and checked. No change. Then I plugged the right rear, right front, and re-bled the left front. Still no pressure. I figured it had to be the master cylinder so I replaced it. With only the left front caliper (and the brake light switch on the right side of the master cylinder circuit) connected, IT STILL WOULDN'T WORK!
Checked the blog and found what I needed: "MAKE SURE THE BLEEDERS ARE POINTING UP". Uh oh! The rear calipers were fine but the front were wrong - bleed valves were on the bottom. Somehow I had swapped the right and left front calipers when I rebuilt them. Changed them, reconnected the rear calipers, power bled everything, and voila - I have brakes.
The posting was from 2002, but the information was still relevant 21 years later! Thank you Pelican Parts for maintaining the site.