So, came back to land, fired up the 76 and no back brakes.
Figured they had some air in the lines.
Jacked up the stern of the truck, spun the rear wheels by hand, and my 9 year old hit the brake pedal, no joy.... didn't even slow them.

Wheel cylinders appear clean so no leaks there, no other visible leaks or moist spots on the brake lines.
Tried to bleed them with vac pump, got some froth. Checked out the proportioning valve, held the piston out with a pair of vice grips and hit the brakes, got some fluid, didn't last long. Brakes were stiff before pulling the piston, now slightly mushy and lil slow to come up 100% (first 90% is fast, last 10% takes 5seconds).
These are NOT power brakes, so there is really just the MC, Pvalve, and front discs/rear drums.

So I am concluding a proportioning valve problem, any other suggestions before I go to the junkyard?

as always appreciate any help.... always nice to have a few more heads in on it.
Drew

 

I would suspect the master cylinder before the proportioning valve.

 

X2. You said the truck's been sitting a while?

 

thats interesting.
why would a MC work on one section (the front) and not work on the back?
right now, I cannot get fluid flow out of the valve to the back brakes but I can the front.
thanks tho, you got me thinking, I'll swap the feeds for the front and back around to see if I can get fluid to flow.

thanks,
Drew

 

X3 on the MC, do yourself a favor and spend the extra 20-25 bucks on a new one if you only want to do this job once, too many issues with the reman'd MC's.

 

yeah, I bought the truck and It was sitting probably for 4-5 years.
cleaned out the fuel tank.
I got a 4v intake, took about 4 carbs and made a good holley 1850, got the exhaust, headers, and all installed/working just peachy.
Basically took care of what needed to get the truck to run.

took it out of the yard to road test in a unpopulated area, and noticed the brake malfunction and that the truck wandered a little.... so working those issues out.

like I said, swapping the lines front to back should show me what I need to know.

thanks,
Drew

 

dang you guys are fast.... I can't even post and new stuff pops up :-)
considering this is a fun truck/money pit, I have no problem throwing a new MC at it just cuz.... (I mean, it IS the brakes)
I'll grab a MC in the morning, test that out, and see if it solves the problem.

I will mention again, the front brakes are great....

 

The piston in the MC that forces fluid to the rear brakes may have failed but the fronts would still be operational.

 

That's the point of a 2 reservoir MC. It's like 2 seperate systems. If one fails, you still have some brakes.

 

roger roger.

 

just as an update for the sake of closure.

Took apart both MC and Proportioning valve. Rusty sludge in the both of them.
Cleaned all 35 parts in the prop valve, sat the body in carb cleaner and cleaned/blew it out.
MC front brake piston looked perfect, but the forward piston (for back brakes) was seized in place and I couldn't get it out. So got a new MC and everything is happy.

tho spent a ton of time cleaning/blowing out all brake likes and inspecting calipers/cylinders for more trash (hate to just be moving trash further down the line)


Thanks alot again, 8 heads are better than one!
Drew

 

Glad to help, happy that it all worked out for you and you're back on the road.

 

I had a hunch that was it, glad you're back on the road.