Hey All:
So I replaced my Brake Master Cylinder in my 88. I thought It went well But after bleeding the brake and cylinder system, I have no brake pressure. I have no idea what the problem is. There is pressure when the car is off but as soon as I turn it on...nothing. Any Help to get my truck back on the road would be Awesome!

So I see after reading many other posts, this is not an uncommon problem. I guess I'll just keep at it. My only question... where and what is a LSVP. I must have missed that step. My manual only says to have it repaired by a professonal. Which I am not. I am only a chick that likes to buy old cars that need love. WHY OH WHY DO I FALL FOR YOU EVERYTIME!!

Load sensing proportioning valve is suspended near the rear axle, there will be a cable that goes from rear wheel to wheel and it is located somewhere along that cable. I have never had to mess with one so I am not that familiar with it. I don't really think that is your problem but it would be worth investigating.
Good Luck

The LSPV is located on the passenger side frame above the rear axle, and a thin rod connects it to the rear axle (driver side). When there is a lot of weight in the rear of the vehicle it sits lower, so the angle of the rod changes, and more brake fluid pressure goes to the rear axle. It needs to be bled along with the 4 wheels. The proper order is:
LR
RR
RF
LF
LSPV

That kind of sounds like a problem with the brake booster (the big donut like thing the Master Cylinder bolts to). Is the vacuum line connected to it?

I'm seconding the brake booster. While you should still re-bleed if you didn't bleed the LSPV, the fact that power falls when the engine is running indicates a booster issue.

I thought about the booster. I tested it by turning the truck on/off then pushing the brake. The pressure would build and hold, so it is deffinetly air tight. The lenght on the push rod seem correct. I can hear it pushing the piston in the m/c. However. When I pulled out the rod to adjust the nut, it slid right out to the end of the booster. Nothing on my 21 year old truck moves that easily, so I wonder the push rod is working properly? I also saw scoring on the rim of the new m/c piston. I am wondering if it is connecting completely. I read that the rear of the truck may need to be elevated to level the m/c. Anyone ever done this to mount a M/C? Still to me, none of this explains why when I turn on the truck, the pedal goes right to the floor like there is absolutly no compression. The cost for a booster is around $200 Yikes! Anyone know if it can be rebuild. I'll check the hoses and I found the LSPV thanks!

Did you bench bleed the MC before installing it? that's an important step.

Well I attemped to bench bleed but I do not have a vice grip and I was not able to get the piston to push. I think sometimes I am a little timid when I'm not sure if I am doing something correctly.

I think we gottem! Because I didn't bench bleed, I tried gravity bleeding. With all valves open, I let about 1 1/2 resivores of fluid run through the system. Finally, with the car running I was able the bleed out the system. Hitting the LSPV help out a lot with the rear. Pressure is finally better than before replacing the MC. I think I'm going to hit it one more time all the way around, then take him for a spin.