front brake lines refuse to bleed '89 half ton
 

I have spent the last six days trying to bleed the brakes on my truck and have so far been unsuccessful. I needed to flush the brake fluid because it was black and for some reason decided it would be a good idea to rebuild the mc as well. I got a rebuilt mc from kragen for $45 and proceeded to bench bleed it and install it on my truck. I started bleeding the brakes with the simple pumping of the brakes method (i've bled my motorcycle brakes this way, and a friends celica) pumping a few time on the pedal, holding it down and at the same time cracking a bleeder screw for a moment to allow fluid to flow ( i did LR,RR,LSPV,RF,LF). I got the rears to bleed fine and when i got to the LSP valve a few bubbles came out and a tiny trickle of fluid. (i have the end of the bleeder tube under fluid as well to act as a check valve so that air can not be pulled back in) I move to the front two brakes and same thing only a trickle of fluid then nothing.

So i switch to a power bleeder and with a compressor and regulator pressurize the fluid reservoir to 10psi. Now the rears bleed easily, much faster than the pumping method. But still no luck from the front or LSPV. I loosen some of the lines to see if fluid will come out of the front lines and it does. So i think it is the bleeder screws, so i take them out and clean them and make sure i can blow air through them and reinstall them. While they were removed, no fluid flowed out of the calipers. I crack open all three valves, LF,RF,LSPV and suddenly it starts to bleed. So i bleed the fronts and they come out clear but the LSPV is still a trickle and won't bleed fast. Then all of the lines once again refuse to bleed. I read some stuff about bypassing the LSPV and i would need two 90 degree elbows with 10mm -1.0 inverted flair threading, but i cannot find anyone one who sells them, or even metric threaded brake lines for that matter. I called up like 10 specialty shops in the bay area and no one has metric thread brake lines only 3/8 NPT. I search online and find metric thread plugs and straight unions, but no ebows. Next I try to find the location of the blockage and every line i open up from the front brakes is now mysteriously dry and yet another thing the master cylinder is frozen and i cannot pump the brakes.
So i remove the master cylinder and find that the second inline piston has gone past the reservoir inlet and not returned, which might have been why it no fluid came out. With the master cylinder off, i clean all the lines and make sure i can blow air through them. Then i return the master cylinder with the jammed piston to kragen and get a new one, bench bleed it and install. This time i start with the power bleeding method. Rears again bleed fine, LSPV gives me a lot of bubbles and a trickle of fluid and the fronts give me nothing. I crack open the lines connected to the mc and find that when the reservoir is pressurized it somehow forces the second piston for the front brakes past the reservoir inlet so that no fluid can flow out the front lines. When i unpressurize it backs off and fluid can flow out of the front port. I talked to a mechanic friend of mine that work at a toyota dealership and he said the power bleeder is the same method they use at the shop, so he is not sure why it is happening. So i decide to to try the pumping method again, because apparantly i cannot power bleed the front brakes. I get a few more bubbles from the LSPV and a few small ones from the LF but then again, nothing more. I open up the line coming from the front brakes to the LSPV to see if it is something from the internals and nothing comes out of that line. And now when i am pumping the breaks i hear what sounds like bubbles in the MC, i had previously tightened all lines and checked for leaks with a little oil on the threads and found nothing. So i don't know how the air got in there.
I'm about to break down and take it to midas or just rip out the OEM system and replace it with aftermarket cause i think this dual piston MC is part of the problem, cause its been out for 6 days and it's my daily driver. But brakes, at least when i've worked on them before have always been simple, just a pump that pressurizes fluid in a closed system.
Any insight into this problem would be much appreciated.

How far in are you pushing the MC piston in when you bench bleed it?

until it feels like the piston hits a mechanical stop. and i continue doing it until the fluid recycling back into the reservoir through clear lines contains no air bubbles

Well, I can't say it's "definitely" your problem, but I've learned to only depress the piston no more than one inch as any further could damage the seal. That's what I've read and been told before when I ran into the same problem. However, I've also read to the contrary....push the piston all the way in. So, it's a bit confusing to say the least....can't explain the discrepancy. But, I can tell you when I only partially depressed it, I had better results.

I went through a similar problem with a friends truck started out bleeding the whole system 4 times and the rears would only seep a little same as the front.
the po to him had removed the lspv.
we ended up replacing the MC, rear brake pistons and the front calipers out of it
in my own quest to find out why they where not working I took apart the calipers and the rear pistons and they where full of sediment to the point that they kept clogging up the bleeder valves.

when I got my 4runner I rebuilt my calipers with a napa seal kit that cost me 10$ a wheel and I have had no problems it did take some time getting out the pistons and cleaning them and polishing them.

that why with my buddys truck I just had him buy new ones I did not feel like spending the time on rebuilding them.

Good point Cymon...

If the fluid's black, likely there is sediment. That would certainly cause problems.

it was running clear on the truck I was working on after 4 bottles of fluid thats 4 quarts we used but it was still flowing really slow and we had no pedal that is when I made the call to replace the stuff we did.

you might want to replace or remove the rear brake pistons and back flush them.

the one thing I have noticed with these trucks brakes is sediment stays in the bottom of the calipers and rear main cylinders due to the bleeder being on the top.

the bleeder on top dose make easier to bleed and keep air out but stuff collects in the bottom.
when I rebuilt my calipers there was a lot of gunk in the bottom and I was glad that I had taken the pistons out and cleaned it out.

i went to napa today and purchased the rebuild kits, they should be there tommorrow, and i am taking the front calipers off tonight and cleaning them.

I hope i don't have to do anything with the LSP valve, cause i would probably replace it with the wilwood manual pv but then i would need some break line adapters and no one in my area carries 10mm-1.0 inverted flair in stock, so it would be several days of waiting.

to properly bleed the lsvp you need to do it from the return line at the front of the truck after bleeding the valve it self.

i just took apart my calipers and actually they are remarkable clean inside, pistons were shiny, no sludge. just a quick spray of brakleen and they were spotless.

Here is a diagram of my master cylinder.
http://static.howstuffworks.com/gif/master-brake3.jpg

now for some reason when i pressurize the reservoir with 10psi shop air, that secondary piston compresses and goes past the inlet from the reservoir so that fluid can no longer flow from the reservoir and out the secondary outlet. As soon as i release the pressure the piston moves back and fluid can flow again.

Pressurizing the reservoir is the normal way most shops bleed brakes, so why is pressurizing the reservoir making it impossible for me to bleed my front brakes?

I have never used a power bleeder, but my first guess would be to try less air pressure. It probably only needs a few pounds to get the fluid flowing.