Power booster?

[chengny]

"Brakes stop the truck but with a little effort and low pedal. Adjusted back brakes up snug, ebrake good, and have re bled."

As 75K30 states: "This device only provides assistance and only makes it easier to apply the brakes. It has not affect on the travel of the pedal, nor will it make the pedal spongy if the booster is failing. Matter of fact the symptom of a bad booster is a hard to push pedal."

Automobile brake applicatons are a relatively simple, straight forward hydraulic system. If you are confident that there is no air in the lines and that all pistons (4 slave/wheel and the master) are well sealed up against their respective cylinder walls (and a leaking wheel cylinder can be easily identified as the fluid can only leak onto the ground) there must be a place that the pressurized fluid is flowing to, other than the wheel cylinders.

A single stroke of the master cylinder piston within the cylinder can only displace a pre-determined volume of fluid (stroke length times piston area). Due to design (check valves and orifices), pumping the brakes multiple times will eventually allow the system to come up to required pressure even if there are pockets of air (or if there is air entrained in the fluid). But this is not what you want - right? Most drivers prefer to feel significant brake application upon a single touch or pump of the pedal.

To my knowledge there are no loops or by-pass devices in these trucks. The only component of any sophistication is the proportioing valve - I believe its called. This only serves to direct more fluid under pressure to the ... here Alldata explains it better than I can:

These vehicles use either a two function or three function combination valve. Some vehicles use a height sensitive proportioning valve in addition to the combination valve.
The two function combination valve consists of a metering valve and a brake failure warning switch. The hydraulic brake lines are routed through this valve to the wheel cylinders or calipers. The metering portion of this valve assists in providing balanced front to rear braking by delaying full hydraulic fluid pressure to the front disc brakes until the rear drum brakes overcome return spring tension and the linings contact the drums. The brake failure warning switch portion of the valve activates the brake warning lamp when there is a loss of pressure in either the front or the rear braking system.
The three function combination valve consists of a metering valve, a brake failure warning switch, and a proportioning valve. The metering valve and brake failure warning switch operate the same as those in the two function combination valve previously described.
The proportioning section of the valve proportions outlet pressure to the rear brakes after a certain rear input pressure has been reached, preventing rear wheel lock-up.
The height sensitive proportioning valve provides ideal brake balance according to weight at rear axle. This valve is mounted on the frame and responds to changes in vehicle trim height in relation to rear axle load. Mechanical linkage connects the valve to a bracket attached to the rear axle.

But the point is there is no where for the fluid to go other than to the wheel cylinders. Have you had control of the truck for a long time or did you just get it? Were the brakes spongy before you went through the system? Is it possible that some previous repair was done in a less than professional manner; i.e. did someone use low pressure hoses to patch a rotten section of line? If under-rated hoses are inatalled they can and will expand like a balloon upon application of brake pressure. So, when you press the pedal the fluid expands the rubber hose rather than applying pressure to the slaves.

Just some food for thought - good luck.

Wow, that went long - sorry.

[wallybay2008]

Thanks again for all the feedback. When I get home on the 10th I will rebleed the brakes again (4th time). I did have a helper (experienced) and we were thorough using a clear bottle and looking for air bubbles. Did 3 pumps after air bubbles stopped and never saw bubbles the next 2 times. So I'm fairly sure no air. BUT, never know. I will also be testing vac pressure but I am almost convinced since the master was leaking into the booster and the truck was little used for 2 years, that the diaghram is toast. What do you expect for $800 with a couple year old 8' fisher plow.

[wallybay2008]

Thanks Chengny, I am also thinking proportioning valve if the booster is not the culprit. But the booster is the cheaper of the 2 if guessing. $100 vs. $300 Also I did just get the truck and the brakes were like this before I started. Will also check for ballooning in all hoses. Never heard of that before, but makes sense.

[wallybay2008]

Hi Everyone,
Sorry it took so long to follow up but I had to work over.
Problem was combination of proportioning valve and air still in system. Actually you could chaulk it all up to air as the proportioning valve was air bound and I had to bleed that first. Then do everything else in order. Was told this by an ex toyota mechanic, go figure. But in reality it was air.
Thank you to all who helped.
Wallybay2008

[leolkfrm]

air can be hard to chase, glad all is well and thankyou for posting the results

[chengny]

"If you are confident that there is no air in the lines... blah blah."

Good luck, these are nice trucks.

[gmctrucks]

Originally Posted by wallybay2008

Hi Everyone,
Sorry it took so long to follow up but I had to work over.
Problem was combination of proportioning valve and air still in system. Actually you could chaulk it all up to air as the proportioning valve was air bound and I had to bleed that first. Then do everything else in order. Was told this by an ex toyota mechanic, go figure. But in reality it was air.
Thank you to all who helped.
Wallybay2008

Wally im glad it's fixed I was thinking it was air. I once heard that some guy placed on new rear wheel cylinders up-side down so that's why he couldn't bleed it correct I forgot in what type of vehicle it was. Now thinking about it I think it was a Ford explorer or one of them type of SUV but hang out you see we don't know everything so it might be you helping us out or just hang with us.
Take care! GMC