It wouldn’t matter if only one of the piston seals is bad. The hydraulic pressure has to be equal between both circuits, otherwise the fluid would flow back into the reservoir through the port of whichever circuit has the bad seal, and the pedal would sink.

I should’ve clarified my earlier statement as well - it’s not that the MC couldn’t possibly be the problem, but since the pedal is firm with the ports blocked off, it’s unlikely the MC is to blame.

 

Got a chance to take another look at it today. The front small section of the reservoir was almost empty. I topped everything off and pinched the rubber hoses off one at a a time. It seemed maybe like it would pump up quicker with the passenger side blocked off. When I started bleeding the calipers again, the small section of the reservoir would get really low again. Isn't that supposed to be for the back brakes? I don't see any leaks anywhere in the lines. There is dampness around where the reservoir meets the body. The seals are curled up on the edges.

 

Is it possible that this truck needs to be hooked up to a scanner to bleed the brakes?

 

A quick question, are the calipers on right? Bleeder at the high point of the caliper. The calipers can be put on either side and if put on the wrong side the bleeder will be low and you will never get them bled because air will be trapped above the bleeder.

 

I've done a bunch of breakwork on my POVs, is this probably the original master cylinder? I've found that the originals tend to fail when bleeding, probably knocking loose the structural rust and goop...

If you disconnect one of the brake lines and can gravity bleed it (showing there isn't a blockage) the next thing id do is try to pump it in to a container while disconnected. If you don't get a massive rush of brake fluid, then mc is probably toast.

Firstly though, are you following proper bleeding procedure? Closest to MC to furthest away?

 

Furthest away first, working towards closest is the correct procedure.

 

yeah what he said! Whoops

 

Losing fluid that quickly would be seen more clearly than a little dampness.

Check the rear drums for signs of fluid leaking at the bottom - if one of the wheel cylinders is leaking, it will run out the bottom of the drum and pool around the wheel.

The rear line runs along the driver frame rail and above the gas tank - it’s very common for them to rust out near the gas tank without leaking badly enough to spray fluid everywhere, so inspect the frame rail for wet spots. If there’s a tank shield it may be helpful to remove it.

A scanner would allow you to bleed the ABS modulator valve, but it would require a Bosch Tech 2 or equivalent - if you’re going to buy something, make sure it’s capable of performing specific functions. Expect to spend $500 or more.

The general rule is that the ABS doesn’t have to be bled unless it’s been taken apart or replaced and air has been allowed to enter it. That said, I’ve found it helpful on some trucks to perform an ABS bleed after brake work and it helps stiffen the pedal.

 

Thanks everybody for all the great info. Little Bear, you were right, I had the calipers on the wrong side. Not sure why I didn't double check part numbers. Great learning experience.
Merry Christmas!