1980 e12 no brake pedal response

[nerdorama]

Hi everyone,
I recently installed a new master cylinder and now I can't get any pedal response. It's soggy without the car running, but once I start the motor it just goes to the floor. I've used a pressure bleeder. I get good flow to all six front bleeders. The rears flow very sluggishly but when I crack the fittings in the lines it seems to flow just fine. It appears that the rear pressure regulator is leaking. Could this cause the lack of pedal even though the fronts seem to be ok? I've searched the archive but don't find anything that quite matches my perceived problem.
Thanks,
John

[Mike W.]

If it's leaking, it can suck air back in too. Also where are you cracking the lines to test, at body or at the caliper, because rubber lines can get swollen and now allow flow thru them. It's a non issue if you're checking at the caliper and get good flow, but if it's on the body side it could be the problem.

[nerdorama]

Also where are you cracking the lines to test, at body or at the caliper, because rubber lines can get swollen and now allow flow thru them. It's a non issue if you're checking at the caliper and get good flow, but if it's on the body side it could be the problem.

Hi Mike,
Thanks for the reply. First I cracked the fittings at the entry to the rear calipers and got what seemed like decent flow. The rear calipers are older so may have some sludge or muck. I pulled one of the bleeder fittings completely and blew through it to make sure it was clear. I got a little muck from it but could still get air flow ok.

Could the pressure regulator be causing the soft pedal or restricting the flow to the rears? I think I've got to order a new one anyway to solve the leak. They seem to be somewhat harder to get and not too cheap. A couple of online sellers say they are in stock at around $150. The local BMW dealer says only one in US stock for over $200.

It's funny. The car seemed to be working fine and then it sat for about 10 days during a vacation. The brake pedal went to the floor on my return. Seems a bit odd to me but old stuff has a way of going wonky.
John

[RonDwyer]

I would replace the hoses front and rear, fix the leak and pull the bleed screws completely out of one of the rears, then test it again for flow. No flow, you likely have a crud block. When was the last time a rear caliper was rebuilt or replaced? I cannot stress enough how important it is to just get over the notion that 15 or 20 year old calipers are fine. They cannot be expected to last that long especially in wet climates. I regularly see an accumulation of black tar inside, a combination of fluid that has mixed with water. It does not flush if it has sat for years, it just allows the pucks to rust and bind up. Bad seals let in water and will rust the pucks above the bore seal. Binding then happens.

Depending on your skill level, splitting them and doing a proper overhaul with a brake home chucked into a small drill press is a messy job, but you'll have a brake system that performs like a new car. Conventional rubber hoses from ATE are just fine, are DOT rated and safe for daily driving. Teflon hoses are not DOT approved and if flexed too far the inner teflon tube will crack. I worked for Aeroquip, we know these things.

[PatinaBeforePolish]

Is there a process to prime the master cylinder? I've never done one of these but have always done it with new units for other cars.

[Mike W.]

Is there a process to prime the master cylinder? I've never done one of these but have always done it with new units for other cars.

Just crack the fittings at the master to make sure you are getting flow out of them.

[Lock]

I didn't prime my new master cylinder, just filled it up and bled the brakes as normal. A few bigger air bubbles as expected but all was fine.