Brake problem 911 911 911

[Live4Boost]

Sorry this is reposted from the brake section but i really need help bad

I just installed vacuum brakes on my 87 gn and now i am having problems with the breaks. I have the s10 drums in the back with soft shoes. When i first hit the break it feels like it hits something and then will go down towards the floor more. Also i can't hold any boost at all. All the lines are bled.

Please help

 

[marred4life]

make sure you dont have any leaks in the brake system,(lines, hoses, etc..) try gravity bleed and pressure bleed i usually do the pressure bleed first, make sure your using the right fluid, how long ago has it been since the brake install, its recommended to adjust your brakes from time to time so that the sureface of the shoe get an equall grip with the drum. S-10 drums do you mean the wheel cylinders? but most importantly check for leakss :biggrin:

 

[Live4Boost]

no leaks it's been bled ALOT installed everything about 4 hour ago

 

[Live_to_die]

Sounds too me like you have air in the system still, try to re-bench bleed the master clyinder. And then re-bleed the entire system. I have had similiar issue's with air in the system.

 

[marred4life]

true try the master cylinder again

 

[Live4Boost]

We re-bled everything and changed the proportion valve and still won't hold boost. We popped the cap on the reservoir and had someone pump the brakes and there is no pressure at the rear breaks, fronts are fine but nothing for the rears.

 

[QUIP'6']

did you change brake pedal also?

 

[Live4Boost]

yes and it feels like the angle of the pedal is wrong has anyone else had that problem

 

[Live4Boost]

Another problem we found after pulling our back drums off the wheel cylinders are hardly moving. We can leave the drums off and push in the brake pedal and the brake cylinder only moves about a quarter inch.

 

[Live4Boost]

Come on guys help me out i'm trying to make it to the Buick day at Norwalk tomorrow. I'm kinda leaning towards the master cylinder but i'm not sure

 

[QUIP'6']

just keep bleeding them and make sure you push in the nipple on proportioning valve when bleeding and adjust rear shoes till there tight. i can hold 15# of boost off the line at 3/4 pedal on throttle so thats what you should be able to do

 

[ChrisCairns]

It's your proportioning valve.

From your master cylinder pressure is applied via 2 lines, one for front and one for back brakes. These two lines go into the proportioning valve at each end. Inside the valve is a sliding piston. With equal pressure being applied from master cylinder to the prop valve the piston won't move and allows the pressure to travel through the bottom of valve housing and onto each set of brakes. One line goes to the front and splits into two, going to each wheel. Ditto on the back.

If you get a sudden leak at the front brakes for instance, the front "cavity" in front of the sliding valve has little or no pressure. The back cavity still has pressure so forces the piston into the front, cutting off the line from the master cylinder to the front brakes, and cutting off the leak. This leaves you with just back brakes but will still allow the car to stop. The prop valve is a safety valve.

Sounds to me like the piston in the valve has moved and is shutting off the fluid to the rear brakes. You probably forgot to push in the little button on the prop valve.

Now here's where my bad memory comes into play. I can't remember if all you have to do is relieve the pressure by loosening each line going into the prop valve or not. I think there are light springs at each end of the sliding piston, which should allow it to recenter once the pressure had been removed. Then simply re bleed them and remember to hold in the lil button.

 

[Live4Boost]

I did put on a new prop. valve today. So are you saying to loosen all the fitting again then clamp the the spring thing and rebleed??

 

[QUIP'6']

I did put on a new prop. valve today. So are you saying to loosen all the fitting again then clamp the the spring thing and rebleed??

if you put a new one on it cant be the same as factory. it would have to be a brass one they dont sell the factory ones. anyway i put one on last week and i have no fluid to the rear brakes. when i crack the rear bleeders to bleed nothing comes out so i know the valve is tripped. so i was told to crack the front bleeder and have someone spike the pedal 1/2 the way down and tighten up then proceed to the rear the valve should be centered now. wehen you do the rears pump the brakes slow and then push down real slow so you dont trip the valve again. will be trying this tomorrow. GOOD LUCK

 

[marred4life]

so whats the verdict

 

[Live4Boost]

ok that makes good sense. And you are correct the new one was the brass one. I was still getting fluid to the rears though while bleeding. Maybe the valve moved a little and it isnt getting as much as it should though. I will try this sometime today, I am still going to norwalk just without the car. . . .. . .

Thanks for all the responses and help, I hope I can get this problem figured out!

Bob

 

[alec296]

if you poped the cap off master and someone hits the brakes it should spray
fluid up pretty much all over. if it aint doing that try a master.
is this stuff from a junk yard (used) or from a parts store?
andy

 

[ChrisCairns]

I did put on a new prop. valve today. So are you saying to loosen all the fitting again then clamp the the spring thing and rebleed??

I remember years ago someone said because of the "Porosity" in the stock cast prop valve we must get a new one. So a whole lotta people, including me, bought the new brass one. Looking back I think we switched 'em over FOR NO REASON.....lol.

Anyway, If you've put on a new valve and it's still doing it then the master cylinder must be putting out pressure on only one of the two lines out and forcing the piston in the prop valve to move.

I'm not familiar with the vacuum brake master cylinder so this part may not apply to your master cylinder but the powermaster master cylinder has to be bled separately from the rest of the brake system.

Did you bleed your master cylinder?