Broke Brakes

[joshelam]

Hello Marlin Crawlers,

I'm trying to help my Uncle with his Toy. It's an '85 2wd 1 ton. He had a rear wheel cylinder leak and replaced the cylinder. During the bleeding process it seems the master cylinder died (piston seal puked ), so I ordered one for him.

It's a reman, Aisin casting and appears to be an exact match. I bench bled it and it pushed fluid and bubbles as expected from both ports then cleared up and pushed only fluid, so I installed it. Then checked it again with the pedal before hooking up the lines.

Started bleeding the brakes and only the rear builds pressure (D rear, P rear, P front, D front and then prop valve). The pedal builds and holds pressure at about the half way point, enough that it feels like a mechanical stop. When you crack a rear wheel cylinder bleeder the pedal goes to the floor as it should. However the front doesn't build pressure and when cracking the bleeder it does not relieve the pressure from half way down. We've bled through the system several times and have fresh fluid and no bubbles. To get fluid through the front lines we had to open the bleeder, press and hold the pedal, close the bleeder and repeat. That's the only way I've been able to get fluid to flow through the fronts.

In 30 years of replacing/bleeding brakes I've never had that happen.

I checked/compared the actuator rod hole depth from old master and new, they are identical. I screwed the actuator rod all the way in (as short as possible) as well as spaced the master with a couple washers wondering if MAYBE the rod was not allowing the piston to return (preloaded piston). Nothing seems to make a difference.

So here I am, asking for thought and ideas. Bad Master? Some mystery about the prop valve I'm not privy to? Any thoughts are greatly appreciated.

Thanks,

Josh

[gnob]

Are you on jacks or is the rear axle hanging?

Try a thorough bleed at the prop valve (or delete it)

See if you get pressure at the rf T

[joshelam]

Come to think of it the rear was up, stands under the frame, axle hanging. Going to re-bleed with the weight on the rear axle. That would certainly affect the load sensing valve. Good catch. Will report back in a bit. Thanks gnob.

[joshelam]

Did get more air out, but still no front brakes, still no top half of pedal. Bled all 4 again plus the valve. Tried driving it up and down my 1/4 mile driveway several times and pumping the living heck out of it.

Need more fluid now, lol.

[gnob]

Is it making pressure at the master currently? Obviously questioning the front brakes port.

[joshelam]

It moves fluid, but don't know how much pressure it makes....suspect it's not much and may be bypassing internally. Will have to get back to it this weekend, Sunday when I'm off work again.

[joshelam]

To close this out it was the master cylinder. I got a warranty replacement and it functions perfect. Thanks for your input gnob.