Brake bleeding problems

[Ford Mechanic]

So I've been helping a friend with his 88 CUCV Blazer and it's really making us scratch our heads. So for a while he's had a issue where the red brake light came on in the dash if he mashed the pedal real hard. Well the fluid had always been full and we had tried bleeding the brakes with no change. He has ran like it for over a year with no other issues.

So new problem, last week he lost all brake pedal, straight to the floor. No fluid leak or loss, resoviour was full. We tried bleeding them and found it to have very little fluid flow out of the bleeders. I also tried bleeding at the master cylinder with the same result.

So I figured it was bypassing internally in the master cylinder. So we put one on bled it at the master cylinder first and then the wheels, but have found the same issue. Low flow, and no improvement in pedal.

So had anyone else ran into this problem? It still acts like it has a bad master cylinder? It was a new NAPA unit, but new doesn't necessarily mean a good part. So I'm not putting it past another bad master cylinder but think it's just odd the issues we've had. Anyone else ran into this? Suggestions would be much appreciated.

Mike

 

[Coffey1]

Does it have a power booster on it.

 

[Ford Mechanic]

Yeah, a hydro boost system. Sorry I thought they were all equipped similarly. It has the 6.2L n/a engine I think.

 

[Coffey1]

That's your problem. Hydra boost bad

 

[the skull]

Clogged bleeder screws? Collapsed rubber brake lines? Proportioning valve bad?
No offense, did you bench bleed the new master cylinder?

 

[Ford Mechanic]

That's your problem. Hydra boost bad

We were bleeding it with engine off, have you had issues like this due to the hydro boost unit? I've had them fail before on other vehicles and it resulted in no brake boost with a very hard, high pedal.

 

[Coffey1]

On my work truck it would have OK pedal then next time it would go to the floor and scare the crap out of ya when it decided to go to floor at the worst moments.

 

[Ford Mechanic]

Clogged bleeder screws? Collapsed rubber brake lines? Proportioning valve bad?
No offense, did you bench bleed the new master cylinder?

We bled the master cylinder on the truck, pumped it up and cracked the lines one at a time. Got some air out, then started at the wheels in the normal order.

Yeah I haven't removed the bleeders so they could be partially clogged.

A collapsed line should still have hard pedal but have issues with brakes releasing. I have no pedal at all, goes straight to the floor like it's full of air.

Proportioning valve, should still have good pedal also, but either too much rear brakes or too little. I would reason anyway.

No offence taken, it was a very valid point and necessary 1st step. But he already had it on and had no vice at his place, so I just did it on the truck. I still may end up pulling it back off and taking it to my shop to bleed it at a different angle. With no improvement in pedal pressure at all it it feels like it's full of air or bypassing.

It also has a noise when pressing the pedal down coming from the master cylinder. Sounds like fluid rushing inside.

Thanks guys

 

[sweetk30]

gm test for bad hydro boost unit .

pump pedal engine off 10 x . then on 10th hold to floor or down as far as possible.

fire up engine and pedal should drop and then rise back up and over power your foot pressure.

if dosnt do this = bad .

 

[Chaski]

It has a GM combo valve which is a fixed proportion valve and an evil shuttle valve with a switch that goes to the "brake" warning light. You may be experiencing a stuck combo valve. The shuttle valve is supposed to move then block flow to one of the front wheels should a hose get severed. When the shuttle valve moves from it's centered position it activates the brake light switch. It is under the front of the truck near the steering box. Might be worth fiddling with it. I tossed mine and put in an adjustable Wilwood purporting valve.

 

[rustystud]

It has a GM combo valve which is a fixed proportion valve and an evil shuttle valve with a switch that goes to the "brake" warning light. You may be experiencing a stuck combo valve. The shuttle valve is supposed to move then block flow to one of the front wheels should a hose get severed. When the shuttle valve moves from it's centered position it activates the brake light switch. It is under the front of the truck near the steering box. Might be worth fiddling with it. I tossed mine and put in an adjustable Wilwood purporting valve.

This valve will not cause this problem. Sounds like the master cylinder is not picking up the primary circuit ( front brakes). Since this circuit is the last in order of application (front of master cylinder) it is the hardest to bleed out. Especially in a GM system. I would remove the brake lines from the master cylinder and use some tubing to reroute the fluid back into the reservoirs (basically bench bleeding) . Give this a tray before you condemn the master cylinder. If you get good flow then reconnect the brake lines and try and bleed out the rest of the system. From my experience I have found the GM master cylinder to be one of the worst to bleed air out of. Fords, Chrysler, any other vehicle OK ! GM the worst.

 

[Ford Mechanic]

This valve will not cause this problem. Sounds like the master cylinder is not picking up the primary circuit ( front brakes). Since this circuit is the last in order of application (front of master cylinder) it is the hardest to bleed out. Especially in a GM system. I would remove the brake lines from the master cylinder and use some tubing to reroute the fluid back into the reservoirs (basically bench bleeding) . Give this a tray before you condemn the master cylinder. If you get good flow then reconnect the brake lines and try and bleed out the rest of the system. From my experience I have found the GM master cylinder to be one of the worst to bleed air out of. Fords, Chrysler, any other vehicle OK ! GM the worst.

After reading this and talking to a older tech that had experience with these that said the exact same thing, I think we are going to make up a correct master cylinder bleeder and restart from this point. Will keep you posted.

 

[Mainsail]

The last time I bled the brakes on my 1009 I messed up and got a big air bubble in the lines. There was enough air in there to compress and consequently almost nothing would come out the bleeder. The wife was getting tired of playing so I just hooked up the big vacuum pump and sucked the air bubble out the left side aft bleeder. After that it went smooth.

 

[Drock]

One of the first things I did on my truck after buying it and getting it home was to change out the front calipers + pads, and rear wheel cylinders + shoes. For the life of me I could not get the stupid thing to bleed out. So I ordered ah set of Russell speed bleeders. I've had them on all my cars for the past 20 years. They bleed out way better because they have a spring loaded check valve in them which builds pressure as your bleeding, forcing the air out rather then asking to come out. Plus it only takes one person to bleed! Good if you don't want to argue with your wife, and or if your stuck on the side of the road. Here's ah link , you'll have to order the correct size for the front and the back (sorry I can't remember what size mine were)........http://www.summitracing.com/parts/rus-639510?seid=srese1&gclid=CK3Cw9zNmcYCFU08gQodDncAAw

 

[Ford Mechanic]

So we tried again this afternoon, and I think we have a bad new master cylinder. We made up a bench bleeder from some brake line with clear tubing on the end so we could see what was going on. The front resoviour (rear brakes) bled quickly. The rear resoviour (front disc) never would cycle any fluid whatsoever. So I got thinking and hooked the 2 brake lines together with the clear tubing to try and force or pull fluid into from the rear resoviour. Well what I found was unexpected, pumping the pedal slow found that instead of pushing any fluid into the rear or blowing the line apart. Instead it bypassed into the front chamber with very little pressure. We disconnected and reset it for bench bleeding. I re-bled the front and then pinched the tubing again on the front and confirmed that it was again bypassing the front piston. So we've removed it and ordered another. So we wait until Tuesday to replace it again.

 

[Ford Mechanic]

A pic of our bench bleed set-up

 

[rustystud]

So we tried again this afternoon, and I think we have a bad new master cylinder. We made up a bench bleeder from some brake line with clear tubing on the end so we could see what was going on. The front resoviour (rear brakes) bled quickly. The rear resoviour (front disc) never would cycle any fluid whatsoever. So I got thinking and hooked the 2 brake lines together with the clear tubing to try and force or pull fluid into from the rear resoviour. Well what I found was unexpected, pumping the pedal slow found that instead of pushing any fluid into the rear or blowing the line apart. Instead it bypassed into the front chamber with very little pressure. We disconnected and reset it for bench bleeding. I re-bled the front and then pinched the tubing again on the front and confirmed that it was again bypassing the front piston. So we've removed it and ordered another. So we wait until Tuesday to replace it again.

Sounds like someone forgot a seal when they built this master cylinder. Is it a rebuilt ? They could have forgotten the pin that separates the two chambers from each other. This would allow the front brakes to bleed back to the rear brakes. I have seen this happen before.

 

[Ford Mechanic]

No' unfortunately this was a new unit.

 

[sweetk30]

and why Tuesday ? ? ?

if for new to arrive that's bull crap on there part for defective unit replacement .

 

[Ford Mechanic]

The closest one will be there Monday, it was to late to have it today, and their truck doesn't run on Saturday. The guy that owns the truck, it's at his place of buisness and is not open on Monday.

He's lurking here, maybe he'll sign up and say hi.