Pressure bleeding problems

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Psucamper
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Location: Boalsburg PA
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Pressure bleeding problems

Post by Psucamper » Sat Sep 22, 2012 9:09 am

Hello all...I have a 1970 std sedan that has set in a dry garage for more than 10 years without being run (except for engine). Brake system is totally shot. Wheel cylinders frozen and flex lines collapsed. Brake fluid missing apparently due to leaky wheel cylinder. No fluid in steel lines and horrible gunk at the leak. Installed all new shoes, cylinders and flex lines. Now the problem: using the Motive power bleeder, the front wheel hydraulic lines bleed correctly and no bubbles. But at either rear cylinder I get only air and essentially no fluid! This continues after many many attempts. How can this be? The entire system is under 10 psi air/fluid pressure so how does massive air get into the rear system?
Naturally I more than suspect the MC as all the other fluid parts went bad over the years. Are the compensating ports blocked or is there a seal in the MC that leaks? Would think that under pressure that fluid should squirt out of someplace is seals are bad..don't see any. Similarly, if I do install a new MC, do I really need to bench bleed if I use the Motive pressure bleeder tool? Is a puzzlement. Bob L.

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Amskeptic
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Re: Pressure bleeding problems

Post by Amskeptic » Sun Sep 23, 2012 6:08 pm

Psucamper wrote:Hello all...I have a 1970 std sedan that has set in a dry garage for more than 10 years without being run (except for engine). Brake system is totally shot. Wheel cylinders frozen and flex lines collapsed. Brake fluid missing apparently due to leaky wheel cylinder. No fluid in steel lines and horrible gunk at the leak. Installed all new shoes, cylinders and flex lines. Now the problem: using the Motive power bleeder, the front wheel hydraulic lines bleed correctly and no bubbles. But at either rear cylinder I get only air and essentially no fluid! This continues after many many attempts. How can this be? The entire system is under 10 psi air/fluid pressure so how does massive air get into the rear system?
Naturally I more than suspect the MC as all the other fluid parts went bad over the years. Are the compensating ports blocked or is there a seal in the MC that leaks? Would think that under pressure that fluid should squirt out of someplace is seals are bad..don't see any. Similarly, if I do install a new MC, do I really need to bench bleed if I use the Motive pressure bleeder tool? Is a puzzlement. Bob L.
If you have a pressure bleeder, it should be pretty idiot-proof. Check your brake pedal very carefully for complete release from the master cylinder push rod. You should have 2mm click before pedal push rod contacts the piston inside the master cylinder.
Keep reservoir full. Do not let the rear of the car be jacked too crazy high.
5-7 psi should easily drive fluid to right rear cylinder. You might be keeping the bleeder open for up to 30 seconds before fluid makes it back there.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

Psucamper
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Location: Boalsburg PA
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Re: Pressure bleeding problems

Post by Psucamper » Tue Sep 25, 2012 12:38 pm

Colin...checked the clevis push rod to MC and the proper clearance was there.Tried another few pressure pumps. Still no fluid at the rear cylinders. Removed the MC and pushed on the piston. Big squirt from the front circuit line but nothing from the rear circuit outlet. Fits with previous observations. Conclusion:all hard lines OK with no blockage. Front MC circuit OK,back MC circuit dead. New TRW MC ordered. Another damned MC install...Phooey. Bob L.

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Amskeptic
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Re: Pressure bleeding problems

Post by Amskeptic » Wed Sep 26, 2012 7:59 am

Psucamper wrote:Colin...checked the clevis push rod to MC and the proper clearance was there.Tried another few pressure pumps. Still no fluid at the rear cylinders. Removed the MC and pushed on the piston. Big squirt from the front circuit line but nothing from the rear circuit outlet. Fits with previous observations. Conclusion:all hard lines OK with no blockage. Front MC circuit OK,back MC circuit dead. New TRW MC ordered. Another damned MC install...Phooey. Bob L.
There is a bench bleed process. I try to dribble some fluid down in through the reservoir grommet hole while gently moving the piston with a 3/8 extension against my stomach (who has a bench vise where I go?). This "wets" the seals and gets the pumping action going. Sometimes, you have to go through the entire front piston's stroke before you actually engage the rear.
Good Luck!
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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