Brake bleeding problem
 

Just putting my car back together after a repaint. During the repaint I took the opportunity to change the brake flexible lines and renew the caliper seals.

I went to bleed it today. I'm bleeding it by pressurizing the system with 10psi. Bled all four corners 3 times and used about 1 litre of fluid thus far. Yet I still can't get a pedal. I get occasional microscopic bubbles in the fluid, but very few and was surprised how quickly the fluid came through when I first bled it.
When I depress the brake pedal I don't feel any resitance on the first pump or two, then I get a slight resistance build up, but no pedal.
It occured to me it has been sat for maybe 6 months with no fluid in the system (didn't occur to me at the time sadly). That said the master cylinder appears to sit at or below the level of the calipers, so I'm wondering if it would have retained fluid, as when I removed the calipers I was surprised by how little fluid came out.

So, I'm wondering if the seals in the master cylinder have dried out and it's now not holding pressure.
If so would it leak fluid? Can it be visulay inspected to determine if it now unservicable?
If one of the calipers weren't sealing would I find evidence of fluid anywhere?

Any help would be much appreciated.

Adam

Brakes
 

Leaks from calipers will show up on inside of wheel rims.
I would look closely at master cylinder.
Also I would use two-man method. for bleeding. You can better troubleshoot pumping the pedal and observing caliper.
Personally, I use Speed Bleeders (bleed screws with check valves). It's a one-man operation and you get to exercise pedal (and thus master cylinder). I have great success with these on all my cars.

pump the brake until you get some pressure. I may take half an hour of pumping. Once you get some pressure take it for a drive even with spongy brakes. The vibrations and heat will cause the pedal to firm up.

Chris
73 911 E

If your master cylinder is original, go ahead and replace it. Ask me how I know...

Thanks guys. I shall have someone pump the pedal and observe the master cylinder. And I'll check the caliper for signs of leakage. I have a blead tube with a non return valve, I'll give that a try too.

Adam

What can happen is the seals in the MC go South, and you can pump til the cows come home and the thing leaks past the seals internally and won't build pressure. Or in my case, it would make pressure until you started the engine and the vacuum assist came on, and then went totally soft. Very annoying failure mode to chase.

The reality is they do fail and that is one component of a system where failure is not an option. I would replace and not waste another minute.

Sounds like a lot of air. One thing to check for that happens once in a great while. Check that the bleeder screws are pointing up. You can put a right side binder on the left side and the bleeder will point down. Never get the large amounts of air out in this case.

Calipers mounted upside down/wrong side? Bleeder valves should point up.

You replaced the caliper seals? Did you pull the pistons and replace the o-ring? If so, it can take some time for the caliper pistons to adjust.

+1 for a two men method, I had a similar problem I left mine open for a few days, it took me good couple of hours to get the pedal feel.
good luck
just keep pumping....

One method to try is to have have one person pump the pedal while the other holds the bleeder open (usually do this on both rears when starting with a dry system), then bleed it with the power bleeder at 15 psi. Use a clean container and you can re-use the new fluid. Also, as BGCarrera32 mentioned, the seals in the MC can deteriorate and allow fluid to move between the chambers without any external signs of a leak. Good luck!

It's reassuring to know that I'm not the only one who has done that!

JB

There are lots of threads here on brake bleeding. I went through something similar and the threads had some helpful suggestions.
In the end I used a pressure bleeder and got all the air out that I could using that method, but still had a low, spongy pedal. I took each brake pad out (one pad at a time) and replaced it with a piece of 1/4 plywood. I pumped the brakes to force the piston out, then took the wood out. Then I opened the bleeder and pushed the piston all the way back into the caliper, forcing fluid and a small amount of air out the bleeder. This gave me a firm pedal, but it was still low. I did the whole routing again, but this time I only pushed the piston back in far enough to allow me to force the pad back in place with some effort. I had great brakes after that.

Pumping the pedal through its entire stroke will surely destroy the m/c seal, if that isn't already the problem. 1/2 strokes only.

Ok, I've bled the system several times today. I put about a litre of fluid through it until it was nice and clean. I first bled with the pedal. I used a non return bleeder, as knowing how long these things take I couldn't ask someone to stand around hours on end.
I bled the whole system four times on the pedal and got to a point where I would put about 250 mil of fluid through each caliper to ensure no air remained. I then bled it 3 more times by pressurizing the system.
I now get a pedal if I pump maybe 7 or 8 times. The pedal hardness dies off quite quickly.
I just checked out the prices of master cylinders and they are not cheap so I really want to be sure before I buy a new one.

Any recommendations on how to test the MC if I take it out?

I guess I should give the pad removal process a go first maybe.

Thanks

Adam

change the MC. all it takes is one pump to ruin an old one. ive been were u are and the fix is a MC. or u can keep doing it your way and end up insane.

That's a pretty good sign that the seals inside the MC are allowing fluid to pass between the chambers. Time for a new MC.

Good advice, especially the MC. Another one I've seen on other cars is an upward bow in a flex line, where the bubbles just sit there and fluid passes under the bubble. If this is the case, have an assistant bend the flex downwards while bleeding.

Something I noticed that doesn't seem right. The fluid resovoir has two outlets, yet the fluid is only diasappearing from one of the outlets. It has a seperator in the middle and when the fluid gets below the seperator the fluid only drains from the right outlet. The fluid in the left section remains at the height of the seperator.

Is this normal?

Adam

Just to confirm; It is OK to bleed from just one bleed nipple on the caliper, as the fronts have 2?