Can't get brakes bled

[1981lt1]

Hey folks.  I have hit a brick wall with my brakes on my 83 C10 LWB.  Everything was working perfectly until I installed a rear disc kit just before Christmas.  Along with the rear disc brakes, I also installed a Master Power disc/disc prop valve.  From that point on, I can't get a firm pedal.  I have power bled the system and manually bled the brakes and I still can't get a solid pedal.  I have gone through 5 master cylinders (I'm not exaggerating) and it's all the same result.  I have bench bled the master every time and it's solid on the bench, but once I install it on the car, the pedal goes all the way to the floor.  I have put the drum brakes back on the car and put the factory disc/drum prop valve back on thinking I can go back to stock but nothing is working.  It's obvious there has to be air in the system but I am getting a solid stream of fluid at all four corners when I bleed the system.  I have purchased a brake pressure tester and I am getting about 500psi at the front calipers.  From what I have read, I should be seeing closer to 1000psi so I am at a complete loss as to what is going on.  What really confuses me is when I press the brake pedal, there is a stream of fluid that shoots up into the rear portion of the reservoir (font brakes).  This usually means there is air in the system, but after going through 5 master's and bench bleeding all of them, I have to think that stream shooting up is supposed to be happening.  Like I said at the beginning, everything was working great until I removed the rear drums and then everything went to crap.

[Shifty]

What master cylinder are you using, and is it power brakes?

[1981lt1]

Yes, it's power brakes.  According to the information I can piece together, my truck is JB3 so I've been using the MC for that, which I believe is 1x1/8 bore.  Yesterday I bought a JB1 MC thinking the smaller bore diameter would help but it's the same problem.  So, as of right now, there is no progress.

[Shifty]

Maybe start here....

https://info.mpbrakes.com/tech-support

[1981lt1]

I have isolated the problem to front calipers or hoses.  I bled the rears first with the MC blocked and the pedal gets rock hard.  I then moved to the front brakes and because I have speed bleeders I open them up and bled them both.  After many rounds of pedal strokes it just keeps pumping air bubbles into my fluid bottles.  I then clamped both my front hoses and my pedal is now rock hard.  My logic is telling me this means there is something going on with my caliper or hoses right???  How is this possible as these are the same parts that were working just fine prior to removing the drum brakes?

[MIKE S]

Are your caliper bleeders on the top or bottom

[1981lt1]

They're on the top.  Exactly at 12 o'clock position.

[1981lt1]

The plot thickens.  Since I kept getting air bubbles on the front circuit, I put the factory prop valve back on.  I blocked the rear brakes on the MC and decided to bleed the front system first.  I have gone through an entire bottle of fluid and I am still getting air bubbles out of the front brakes.  I can see the piston moving so I know the fluid is flowing but the problem is it seems to be sucking air from somewhere.

[VileZambonie]

Can you clarify what you have right now because reading through this is confusing as to what you have going on at this moment?

[1981lt1]

Right now I have the JB3 MC installed, factory prop valve and factory brakes on the truck.  The rear circuit is plugged on the master so I am only bleeding the front brakes (when both ports are plugged on the MC, the pedal is solid).  While bleeding the front brakes, I am constantly getting bubbles.

[Mr Diesel]

I am inclined to think something is not right with your bleeding procedure. Sounds like you are introducing air at the bleeders. I recommend ditching the speed bleeders, I had nothing but trouble with those.

Put on a regular bleeder. Push a clear hose over the bleeder and route it UP a couple inches, then DOWN into a container for old fluid. You can now just crack the bleeder open until fluid and air begin to flow. It will flow UP, then DOWN into the catch canister. Air will naturally be bled out because of the couple inch rise off the bleeder (air goes up and out, but not back down into the line). You can bleed 1 at a time this way or all 4 at once. You can also speed up the process by pumping the brake pedal too, but check the MC reservoir frequently because it will bleed fast.

[VileZambonie]

Stop looking at air bubbles outside of the system as air inside your system. Manually bleed the entire system, from the RR, LR, RF, LF and then gravity bleed it. Make sure the rear brakes are properly adjusted.

[1981lt1]

Stop looking at air bubbles outside of the system as air inside your system. Manually bleed the entire system, from the RR, LR, RF, LF and then gravity bleed it. Make sure the rear brakes are properly adjusted.

I removed the speed bleeders and went slow and steady across all the wheels and I finally have brakes.  They seem a tad spongy when braking hard, but I don't remember if they ever locked up during hard braking.  I've got the rear drums adjusted to the point where the drums have a slight drag when pulling the drum off.  There isn't an adjuster hole in the backing plate so this is as good as I think I can get them.  The brakes are lightyears better than they were when I started this project.  My question to the group is, should the brakes lockup when I slam on them?  My gut is saying yes, but everything about this process has proven my gut wrong.

[VileZambonie]

The goal is to not lock up the brakes but yes you can lockup your brakes if you stand on them.

[bd]

...There isn't an adjuster hole in the backing plate so this is as good as I think I can get them...

Are there knockout windows in the drums?