[spartazoo]

1966 GTO upgrade to dual master cylinder. Fabricated MC lines to new dual line block to work around manual trans clutch Z bar and rod. Everything looks good. Due to manual drum brakes all around, I did not use a proportoning valve. the prior setup did not either.

I cannot get the brakes to bleed. I have bench bled the MC. I have bled the MC on the car. Fluid does not get to the rear bleeders pumping the pedal or with a hand pump brake bleeder.

I am going to replace the rear brake cylinders to make sure they did not get damaged. I also installed a rebuilt rear axle at the same time.

I am all ears to any suggestions.

Thanks!

[VCho455]

My first suspect would be a bad rear hose. Try cracking the fitting at the hose to see if you get fluid there.

My second suspect would be a bad master cylinder.

I had a new master cylinder several years back that was full flaked off anodizing from the piston. Bled fine on the bench but would not pump fluid on the car. After several hours of messing about I pulled it apart and discovered the issue. I returned it disassembled and demanded another pint of fluid.

[Keith Seymore]

Maybe try gravity bleeding in increments: ie, do the fronts bleed? Then - as far as your new distribution block? Then maybe as far as the rear axle split (the single rear line before it goes to LR/RR?

I like the idea of checking the rest wheel cylinders since it sounds like this rear axle is new to you.

K

[Goatracer1]

Had the same problem with a 1962 Bonneville. The rear brake rubber hose was too short and with the rear axle hanging the hose kinked and the fluid would not go through.

[spartazoo]

I’ve bled the master cylinder on the car. Changed out the brake cylinders. Fluid is flowing. But I can’t get rid of the air bubbles in the back brakes. I can’t find any leaks in the fittings.

I’ve manually bled. I’ve hand pumped bled. I’ve air pump bled.

I am beyond frustrated. It makes no sense.

Fronts seem to have bled normally.


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[Sirrotica]

Defective new master cylinder, or a used one, that has the rear most seal able to suck air where the pushrod joins the spool valve. You may or may not see brake fluid around this area. The bore of the master cylinder can admit air if it's pitted as the spool moves over the pitted portion of the bore.

Air can come into the system if the rear most seal on the spool is defective. Sometimes they will leak fluid, but not always. Depends on how the seals are damaged.

I've seen this problem over the years, but it's not real common.

Not saying this is your problem, but it can be a possibility.

[hgerhardt]

Sounds exactly like I experienced last weekend. It was a swollen-shut center rear hose. Couldn't even get a piece of safety wire through the fwd crimped end. Here's what I wrote in the GTO forum: https://www.gtoforum.com/threads/old...7#post-1051047

[spartazoo]

Originally Posted by Sirrotica

Defective new master cylinder, or a used one, that has the rear most seal able to suck air where the pushrod joins the spool valve. You may or may not see brake fluid around this area. The bore of the master cylinder can admit air if it's pitted as the spool moves over the pitted portion of the bore.

Air can come into the system if the rear most seal on the spool is defective. Sometimes they will leak fluid, but not always. Depends on how the seals are damaged.

I've seen this problem over the years, but it's not real common.

Not saying this is your problem, but it can be a possibility.

I will pull it off tomorrow and look. I may just order a new one, Napa can get it same day. When I rebled it tonight, it had air in it again but seemed to go away as I primed/bled it on the car with a re-circulating bleed kit.


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