Bleeding brakes on restored IH truck

[Ethan]

I am in the process (well I have been for a long time now..) of restifying a D-series IH truck. It started life as a 1210 4x4, but I've swapped out the original drum brake axles for a pair of early 80's chevy 1 ton dually axles, with disc brakes in the front. I finally got everything put together, and got to the point where I'm almost ready to move it under it's own power. I bought a new vacuum booster and master cylinder (for the 1210), and I got a proportionaing valve from a Chevy of the correct year and axle configuration. It took me a long time, but I eventually got the brakes bled to the point where I had a nice stiff pedal with the engine off. Problem is that when I turn the engine (392 bored .030 over) on and press the pedal, it just goes down to the floor with almost no resistance. I have bled the brakes a couple more times after this first happened, but it always turns out the same - stiff pedal with engine off, goes to the floor with engine on.
Any ideas on what my problem could be?
I'm also having issues with my clutch linkage, but I need to collect my thoughts and take a few pictures before I ask about that one
Thanks,
Ethan

[Scott Hickman]

Couple of ideas:

1. Always "bench bleed" the new master cylinder before installing or you can also bench bleed with the MC mounted to the booster with the brake lines disconnected from the master cylinder.

2. Your 1210 was originally drum/drum and now it is disc/drum. It would be better to use the same master cylinder, in new condition, as the truck that donated the axles, along with the proportioning valve for the same chevy truck.

3. Be sure the pushrod length is adjusted correctly for the MC & booster being used.

[WRENCH MAN]

Did you get a "NEW"! master cylinder?, or a rebuilt one??

[Ethan]

I believe it was a rebuilt one.

I tried to use the "correct" chevy MC, but I could not get it to fit on my IH booster. Maybe I need to revisit that. I can't remember exactly what wouldn't fit, but I remember being frustrated and then buying the IH part.

Thanks guys.

[Doc Stewart]

Bad MC or push rod adjustment.

[TBAKPhi22]

You need to get a master with the different size reservoirs meant for disc/drums. It has a check valve for the disc chamber so the calipers don't back off all the time. Get one meant for a late 70's early 80's 3500 chevy. The later ones (after mid 80's) are metric, so they wont fit. I have a stock master on a HydroBoost meant for a late 70's 3500, so you would be able to do it (opposite way though).