Brake bleeding frustration

drola · 03-21-2013, 02:02 PM

I am trying to bleed the brakes on a '68 912. I changed mc, brake lines and rebuilt all 4 calipers.
There is no more signs of air in the system. With a helper pumping the pedal as well as using a power bleeder nothing but fluid comes out. However, i can not get pressure at the pedal. After a few pumps it does develop some pressure but after a few seconds i push the pedal and it goes to the floor. While the system is pressurized with the power bleeder it does maintain pedal pressure.
I'm at my whits end with this. What could be happening here?

schumicat · 03-21-2013, 02:16 PM

did you bench bleed the m/c?

Ian Comerford · Ian Comerford's Garage

Master cylinder. This is often the culprit when the main fault is the pedal going to the floor but everything else seems ok.

Ian

drola · 03-21-2013, 02:36 PM

I suspected the mc to be bad so i bought a new one (2nd one).
I read the haynes and bentley manuals and neither speak of bench bleeding the mc. This is a non boosted brake system. I remember a few years back i did the same on a '76 without bench bleeding and those brakes worked great.
So, if there is air in the mc wouldn't it come out doing the power bleeding?

Swoboda · Swoboda's Garage

I had a similar situation, but the pedal was not going all the way to the floor; I had some pressure. Still, the pedal was very far from what it was before my project and very far from what it should be.

You said you rebuilt the calipers. I used four rebuilt calipers, too.

Although I haven't done this yet, I think this is my solution (thanks to the people on this board!).

Best Way to Bleed Brakes?

Good luck!

Lawrence

SC-Alfa · 03-21-2013, 04:38 PM

Did you remount the calipers with blead valves on top? Not doing this is is a common mistake.

Not familiar with 912 calipers, but 911 "S" calipers have two blead valves. That has triped me up in the past.

rennworks · 03-21-2013, 05:05 PM

is the actuating rod the correct length for that m/c ?

DRACO A5OG · DRACO A5OG's Garage

Check t make sure your hard lines are not leaking, not just at the calipers but near the cross member up front.

I was going thru brake bleed hell only to discover my hard lines at the T was leaking, oh the joy

Ian Comerford · Ian Comerford's Garage

If you're not careful when bleeding using the pedal you can damage the mc seals by over-depressing the pedal ie to the floor. Try to only depress it 3/4 of the way each time to avoid this.

Ian

Lukesportsman · 03-22-2013, 12:20 PM

Honestly, why not just bench bleed first. It's false economy to think its easier on car, and then you have too many variables when it doesn't go straight forward.

I've given up bleeding new systems without a powerbleeder. Unless this is or will be your only restoration or vintage car, its a great investment. My favorite is the vacuum style that uses the aircompressor to draw fluid through lines. Fast, efficient and stress free with no mess as a solo job.

Mityvac MV6835 style. Great for quick bleedings between track days too.

drola · 12:37 PM

ok, to answer some concerns:
1) yes all bleeding valves are pointing up. the rear calipers have 2 bleeders which were done according to bentley - outer ones first then inner
2) i have checked and rechecked all lines and there are absolutely no leaks anywhere
3) actuating rod is unchanged so i don't see why it would now be too short
4) yes, i own and used a power bleeder for this. when the system is pressurized the pedal does hold but otherwise it does not
5) if the pedal does not get pumped to the floor it does not build any pressure. pumping it completely a few times does finally build some pressure, at least enough to open a bleeder and then the pedal goes to the floor.
6) i gravity bled all calipers and m/c when installed. even bled m/c at outlet fittings with helper pumping the pedal. regardless, the power bleeder would have done the same when that was done
i'm stumped. again, this is not the first, second or third (etc) time i've done brakes. and it isn't rocket science. there is definitely a common variable because i doubt brand new 2 m/c's are bad

MTemp · 12:57 AM

Not sure if the 68 912 has a single or dual m/c so please adapt this process if necessary. With the single m/c one outlet feeds the LH front (I think) and the other the RH front and both rears. Get a spare calliper bleed nipple, or bend over and crimp a piece of brake line in a standard brake fitting so it's completely blocked. Use this or the bleed nipple to block one of the outlets (I used the RH front/both rears). Now bleed the LH front by your preferred method. If the pedal is hard and the front LH operates properly you now know the m/c and the LH front is ok. Now use the bleed nipple to block off the tee to the rear callipers and check the RH front. Again if OK you know the issue is with the rears

etc etc until you isolate which component is the issue, and you can then address accordingly.

From my experience if the whole system does not bleed relatively quickly then it never will - there is a fault somewhere and you need to use this sort of process to isolate it. Indeed you can start by putting bleed nipples in all the m/c outlets and just bleed the m/c. Big advantage is you can do it with the system in place.

Good luck. It is a thankless task and brake fluid is awful up your sleeve!

Mick

Rod Schneider · 03-23-2013, 01:48 PM

Quote:

Originally Posted by drola

I am trying to bleed the brakes on a '68 912. I changed mc, brake lines and rebuilt all 4 calipers.
There is no more signs of air in the system. With a helper pumping the pedal as well as using a power bleeder nothing but fluid comes out. However, i can not get pressure at the pedal. After a few pumps it does develop some pressure but after a few seconds i push the pedal and it goes to the floor. While the system is pressurized with the power bleeder it does maintain pedal pressure.
I'm at my whits end with this. What could be happening here?

I'm having an almost identical issue. I just installed a conversion kit to go from a single circuit to a dual circuit brake on my 66 911. I've tried vacuum bleeding, pressure bleeding, and the old "push on the pedal" bleeding. The kit came with an aftermarket master cylinder, which I've replace with an ATE master cylinder, no help. It seems to bleed the front brakes OK, but the rear brakes will not build pressure unless the pedal is rapidly pumped 8 to 10 times, at which time a little pressure at the rears is present. The pedal pushrod was cut down per the instructions in the kit, and there is about 3mm of play at the pedal, so the master cylinder is fully seating when the pedal is released. I'm almost to the point of reinstalling the old single circuit system.......

Rod Schneider · 03-25-2013, 01:06 PM

Well, after doing a LOT of bleeding, I finally got a good, hard, pedal.......
For some reason, I totally had a brain fart and forgot that the rear calipers have TWO bleed valves on them! It seems to work a lot better when both sides of the rear calipers have all the air removed. The moral of this story: Check the obvious stuff first!!!

MTemp · 03-26-2013, 12:48 AM

Don't feel too bad Rod. I used the highly structured and systemmatic method I described above to conclude the problem was with both rear callipers. Then when I removed the road wheels to take them off to send them back to the place that reconditioned them ..... there was the second bleed nipple.

Like you say, they bleed really easily when you use the other one as well ....