Poor Braking

[Edahall]

Here's what I've got:
1990 3/4 ton Suburban
Vacuum assist brakes
Larger than stock 285/75/16 Tires
Cummins Diesel which adds 600 pounds to the front
6 Speed Manual Transmission
Vaccum assist operated by vacuum pump on engine

This truck is sufferering from poor braking and I would like to do something before I rear end someone.  The issue is I cannot get the truck to stop fast enough no matter how hard I step on the brakes.  It seems like the stopping distance is about 2 times as long as it should be because of this.  Also, there is no way possible you can lock the brakes.  The pistons are basically are not applying enough clamping force.  I've had to blow through stop lights because of not being able to stop fast enough for the light that turned red.  Any ideas on what to check for?

Also, how many inches of vacuum is needed for the vacuum brake booster diaphragm to work properly?

The pedal does not go to the floor.
The pedal is not spongy.

[VileZambonie]

So you are sure the vacuum assist is working?

What is the condition of the brake linings? Try flushing out the system as well - make sure the metering valve is working. Make sure the shoes are properly adjusted.

[Irish_Alley]

It almost sound like the rear brakes arnt working. Check also your pads when you remove the drum take a not on how hard it slides off

[ccz145a]

Where are you getting your vacuum?

[Edahall]

Where are you getting your vacuum?

Vacuum is coming from the vacuum pump that's run by the engine.

[Lt.Del]

does it seem like you have to press your brake pedal with all of your weight, like standing up on the brake pedal to brake the truck?  If so, it may be your vacuum booster.  There is a cheap check valve that is attached to your booster, that the vacuum line attaches to.  That is like $5.  If it malfunctions, your booster wont work.  
Booster diaphrams do go bad.  I replaced one in my 79 pickup a couple of years ago.  Made a big difference.

[Edahall]

does it seem like you have to press your brake pedal with all of your weight, like standing up on the brake pedal to brake the truck?  If so, it may be your vacuum booster.  There is a cheap check valve that is attached to your booster, that the vacuum line attaches to.  That is like $5.  If it malfunctions, your booster wont work.  
Booster diaphrams do go bad.  I replaced one in my 79 pickup a couple of years ago.  Made a big difference.

Under normal braking, you don't have to press hard but if you need to stop even semi-quickly, you have to stand up on the brake pedal with 2 legs and even then, it is not close to locking up the brakes.

[Captkaos]

It sounds like the pump isn't giving you enough stored vacuum to assist.  Is the pump pulling from a reserviour that the brakes are connected to or is the system connected directly to it?

[Edahall]

It sounds like the pump isn't giving you enough stored vacuum to assist.  Is the pump pulling from a reserviour that the brakes are connected to or is the system connected directly to it?

The system is connected directly to the vacuum pump.

Also, when I push on the brake pedal, I hear a whooshing sound.  Is this normal?

[Captkaos]

So the pump is going to your brake booster?

It whooshes all the time?

[Edahall]

So the pump is going to your brake booster?

It whooshes all the time?

>>So the pump is going to your brake booster?
Yes

>>It whooshes all the time?
No, only as the brake pedal goes down.

[Captkaos]

No as it whooshes everytime you press the brakes.
Do you know how much vacuum you are getting out of the pump?

Have you considered going to Hydro boost?

[Edahall]

I did some tests and I think what's going on is the vacuum pump is not producing enough vacuum.  What I probably need is some type of reservoir.  However, I'm going to ditch the system and buy a used hydroboost unit on Monday.  Now the question is if the Dodge Cummins power steering pump will be up to task in handling both the power steering and brakes.

I'll be glad to get switch over to a hydroboost unit because my 82 Diesel Suburban has this setup and it stops unbelievably fast and the pedal feel is very positive.  It'll stop just about faster than anything on the road from 20-0 mph.

[jaredts]

Hydroboost would be cool, but you could rig up a reservoir in no time just to see how you like it.  If it solves the problem it sure is easier and cheaper.  Like you said the power steering pump you have on there might not have the output to run the hydroboost.

[VileZambonie]

Why not just fix the vacuum pump?