diesel402004

The many years of owning these ive become used to the hard peddle when using brakes repeatedly--100 point turn anyone?--but ive noticed its worse this winter when below 10 (I know hydroboost). Thinking it might actually be the booster this time. It doesnt seem as bad when its above say 20 or so. Ive never had the actual VP go bad but.... checked check valve and it seems to still seal. All other vacuum operations appear normal. Low vacuum light was coming on , but not lately.

thrashingcows

Could be a bad seal on the brake booster. The internal seal on the piston the brake pedal push rod runs into likes to go. Sometimes tough to hear it due to the noise of the cummins motor, but if you run the truck for a minute or so, then shut it off and lean under the dash as you depress the brake pedal you might hear it hissing.

Be good idea to check how many inches of vacuum the vacuum pump is putting out as well.

Jim Lane

I second checking your vacuum pump, you might be using more than you are producing, when it gets down to zero vacuum you will know it, it will take both feet to stop your truck.
I have a vacuum tank I am going to add to mine for more reserve.
Hydroboost is the ultimate fix.

diesel402004

Found out the VP wasn't holding 18 bar vacuum with constant pressure. The oil leak on the VP has gotten progressively worse the last months, so going to try the reseal kit and see if that will make the difference.

Jim Lane

Is this on the 1991 or the 1993?

Your 1991 should have a reciprocating diaphragm pump with 2 chambers, rebuild is only by replacing them.
The 1993 will have a rotary vane pump and is rebuildable.

If it is the 1991, the bad chamber can be bypassed.

Jim5870

Dont forget to check the plastic hose fitting on the booster.
I've seen them crack, hard to notice its cracked.

diesel402004

It's the 93 model. Trying the reseal kit first because the oil leaks have gotten quite bad. If i find the play is too much while I have it apart, I'll go for the rebuild. Already troubleshooted the booster and check valve, they are fine.

diesel402004

UPDATE: After reseal peddle pressure is normal and no more leaks. Vacuum gauge jumped up to 25 bar--previously it slowly rose to 18--right at startup. All internals appeared normal: bushing play etc.

1972RedNeck

738 inches of mercury? Who says we need hydra-boost?

diesel402004

Ya got me Red. The bummer is I put the cone seal in backwards and it's leaking again. Nothing like doing the job twice....

diesel402004

Well the brake saga continues Still have spongy, hanging rear brakes most of the time. When it gets colder the peddle still gets harder and takes more effort. List of fixed or replaced parts so far:
1. RAWL.delete
2. Rear shoes, springs, cylinders, drums
3. Booster
4. Fluid flush and bleeding (multiple times)
5. Resealed and working normally VP

As with everyone else's issues it's only the rears that are problematic. The MC is the only thing not replaced, but issues dont fit normal MC failure?

edwinsmith

Have you checked your vacuum when it's cold/hard pedal? Also could the heater/AC controls be leaking more when it's cold and losing vacuum?

Just my $0.02 worth.

Edwin

diesel402004

I haven't edwin, but the dragging brake shoes (no matter how much I adjust the star wheels) seems an odd symptom of low vacuum. All the controls have always worked normally. This issue has persisted for 5yrs and 100k miles and all those attempts at repairs. I hear about the MC leaking but haven't seen any signs. Could it be leaking internally--not allowing full shoe retraction? Could the proportioning valve have something to do with it? Even though the brakes work normally--front to rear balance.

edwinsmith

You could have 2 problems. The dragging rear brakes could be due to a frozen P-Brake cable or the P-Brake cable adjustment is to tight. Drum brakes will usually drag slightly in my experience. Just enough to make a noise but not really doing anything. The whole thing sort of floats in the drum until you step on the pedal.

The master cylinder could be leaking into the vacuum booster but you'd see fluid when you took the MC off the booster. That wouldn't cause the brakes to not retract fully.

Edwin

diesel402004

@ edwinsmith When I first got this rig I dug into the tight e-brake arm movement as the main culprit (posted on it too), but my eureka moment faded with that "fix" also. The overheated shoes from dragging have caused more than one out of round drum. I have the star wheels backed off now trying to avoid the grabby shoes issue--makes for even more spongy feel on first stops of the day.