Scary brakes/bad vacuum pump?

[TSjodin]

I have a 1994 3500. I have owned it a little over a year and have noticed that
when using the brakes hard, like parking a trailer, that the pedal gets hard and
I loose vacuum, it's pretty scarry to say the least.
If I step on the pedal hard about 10 times the vacuum warning light comes on.
I have checked the vacuum and have about 25 inches of mercury at idle. If I pull
the check valve out of the brake power booster and then put it back in, it takes
about 40 seconds for the vacuum to come back up to 25 inches. When I check for
vacuum at the pump by itself it takes about 3 seconds to get to 25 inches. If I
place my gauge on the system and let the vacuum get to 25 inches, then shut off
the engine, the gauge indicates there are not leaks.
I was thinking that I have a problem with the vacuum pump and since a rebuilt
would cost around $300. 00 I wanted to check to see if this is indeed the problem.
Also are there any alternatives to the engine driven vacuum pump, like a 12V unit?
I understand that the later years came out with air over hydraulic, is this a better
solution?
Would an addition of an accumulator cylinder help?


Any help would be appreciated

Tom Sjodin

 

[nwohsy]

you are not alone. i too am looking for a solution. i have the same problem with my 95 w/145k. i first noticed it a little over a year ago. at that time if i started the truck and put it in reverse to back up a short distance in a parking lot i had almost no braking power, though the pedal had a lot of pressure. now in addition to reverse, i have a similiar problem going forward at slow speed after i pump the pedal several times. i've posted several times to other lists bit never had a response. hopefully someone can shed some light.

 

['956Wheel]

Tom, Sounds normal. The real test is to park it, without braking excessively, then with engine off you should get a few good stops, or an "assisted pedal" out of it before the vacuum depletes. This test will tell if you have a bad check valve and/or leaks. Some guys have put vacuum tanks on theirs to get more volume hench more vacuum assisted braking.

I went through this, and the bros. on the site set me straight. If you repeatedly pump the brakes when coming to a stop the pedal gets hard but does little, one good solid push on the pedal seems to do best.

Mike

 

[Joe G.]

Tom,

If you use the brakes a lot at idle the vacuum pump can't keep up. The brakes are ok, you just have to stand up on them. When that happens to me I just pause for a little and give it a chance to catch up.

BTW. Welcome to the on line TDR!!! #ad
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Joe George
Eureka, CA

'95 2500 CC auto 4X4,3. 54,Combo EGT/boost guage,custom switch panel,PacBrake,TST #5,DTT TC/VB,Automatic motorhome steps on both sides,Foldacover hard bed cover,Cummins chrome kit,Black steel grill guard,Front hitch receiver

 

[MikeR]

Yeah I noticed that too on my '95. If I'm crawling along at idle and pump the pedal 5 or 6 times,then there's no brake force and I really have to stand on it to stop. I checked everything out,then realized the pump can't keep up at idle.

-Mike

 

[Nate Skate]

Have you checked out the recall on the vacuum unit check valve. It is 7 yr. unlimited mileage. Check out the archives. I had air condition switch probs. Had to contact dc to make local dealer fix vacuum pump for free. This recall dealt with anything the pump could effect. I dont know if this will help or not.
Nate

 

[SMorneau]

My 96 does the same thing. Its not scary; just stop doing it. If you want scary, let up on the brakes after you make a near panic stop and your 4-wheel antilocks clunk. If need to use them again right away, because you were wrong and the panic situation was not over, you have no power assist.

I make it a point to remind my wife and anyone who drives this truck that once you hear or feel the antilock brakes activate never let completely off the brakes until you are sure you done with them. Usually by that time they are ready again.

 

[paccool]

Guys, I know I'm in the wrong forum but was just looking around.

My 93 did the same thing. It kept getting worse and was getting the lights on and not much assist when coming to a stop sign off the freeway. Checked the pump and had good vacume at that point. It turned out to be a leaking brake booster diaphram. I replaced the booster and have not had any further problems. Hope this helps.

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Stan
93 2WD extended cab, Banks Power Pack, K&N air filter, PW Injectors,rebuilt and tweaked pump, auto w/3. 54 rear with limited slip, US Gear Exhaust Brake, 3-1/2" Exhaust, Chrome valve covers and heater lines, 5K air bags, Boost/Pyro/Tach Gauges, Green/Silver, new 40-20-40 bucket seats, 151K

[This message has been edited by paccool (edited 05-30-2001). ]