2nd Gen Non-Engine/Transmission Power Brake Vacuum Booster

[Grinder1]

My truck has the classic "falling brake pedal" symptoms. Been that way for a while but hasn't posed any serious problems. More of an annoyance as I keep thinking "man that's just not right". Stopped with engine running, initial pedal push all seems fine. Then pedal will gradually drop if pressure is continued. Hydraulic systems have all been checked and bled. Installed new EGR Carbon Kevlar pads and now pedal drop seems a little worse. Dodge brake tech thought it might be the vacuum booster. How could that be? Power boost is working fine. Then I did a test back at the shop. When pedal is pushed all the way down (engine running) then backed off, I notice some rebound flex on the booster housing and mount . I'm wondering if booster housing has gotten metal fatigued over the years. Has anyone out there replaced the vacuum booster in order to eliminate the brake pedal drop, and more importantly, did it solve the problem?

 

[Gene Heck]

Sounds like you have an internal leak in the master cylinder. It is bypassing around the cups and going back into the resevoir letting the pedal go down. This is not uncommon.



Gene

 

[Joe Mc]

Mine was doing the same - drift to the floor - that's classic m-cyl internal leakage. Replaced the m-cyl few years back - all's well.

 

[Grinder1]

Yep, that's what I thought it was. Replaced master cylinder with a new one and still have the problem. I had thought pressure may be by-passing to the the accumulators in the Hydraulic Control Unit (HCU) for my 4 wheel ABS, so have a Mopar Factory re-built unit on the way (they are very hard to locate). Then I read a thread that a fellow TDR guy wrote about what sounded like a nightmare of never ending parts replacement on his ENTIRE brake system. He even bypassed all his anti-lock components. STILL had the pedal drop problem. Any other ideas?

 

[Gene Heck]

Grinder,



Is the MC a rebuilt? The braking system is a closed system. If you don't have any external leakage showing then either the master cylinder is internally leaking back past the cup(s) or you don't have all the air bled out of the system yet. I wouldn't jump the gun and replace the anti lock valve yet. Sometimes just bleeding the brakes by pumping up the pedal and releasing fluid and air out by cracking the bleeders (the old fashioned way) won't clear all the air out of the anti lock valve. Been there, done that. You may have to take it to someone that has a "power" brake bleeding setup (air pressurized brake fluid tank and MC lid adapter) to force the fluid and air through. Doing this will eliminate that possibility. Then if the brakes slowly leak down go back to the MC. Just because the MC is new or rebuilt doesn't mean it's any good. I hope power bleeding cures your problem but don't be surprised if you are working on the MC again.



Good luck.



Gene

 

[Texis]

Don't mean to get off topic but...

for my 4 wheel ABS

In a 95?? My 97 only has rear ABS.

 

[Gene Heck]

4 wheel abs

4 wheel abs was an option then. My 96 has it.



Gene

 

[LRutigliano]

My truck has the classic "falling brake pedal" symptoms. Been that way for a while but hasn't posed any serious problems. More of an annoyance as I keep thinking "man that's just not right". Stopped with engine running, initial pedal push all seems fine. Then pedal will gradually drop if pressure is continued. Hydraulic systems have all been checked and bled. Installed new EGR Carbon Kevlar pads and now pedal drop seems a little worse. Dodge brake tech thought it might be the vacuum booster. How could that be? Power boost is working fine. Then I did a test back at the shop. When pedal is pushed all the way down (engine running) then backed off, I notice some rebound flex on the booster housing and mount . I'm wondering if booster housing has gotten metal fatigued over the years. Has anyone out there replaced the vacuum booster in order to eliminate the brake pedal drop, and more importantly, did it solve the problem?

My 2002 Durango does this too. With engine running, push the brake pedal and it will continue to fall to floor very slowly.



However, I believe the proper way to test the master cylinder is with the engine off, trans in neutral. Push brake pedal a few times to relieve power assist, then it should come up and hold with light pressure. If the pedal holds, then your master cylinder is not internally leaking.



I have manually bled the system and finally broke down and bought a pressure bleeder (motive power) and it still feels "spongy". If you haven't bought a pressure bleeder yet, you are missing out. Its the best tool in my garage. Bleeding brakes is now a SIMPLE and FAST experience.

 

[jelag]

It's suggested that any brake system should have the brakes bleed once a year to cycle out the old fluid at the caliper and remove the water in the fluid. . We do this on our trucks. .

If you've had someone install pads on your vehicle, and do nothing but compress the piston, and install pads without opening the bleeder, you've pushed all that bad brake fluid back into the master cylinder... . you can later expect master cylinder failure...

This being said, when you can hold your foot on the brake pedal and it slowly drifts to the floor, it is a bad master cylinder and needs to be serviced or replaced... think about this. . how do you know that in a panic stop, and you put all your weight on that pedal that the seals (cups) in the master won't just give up and fail, and you've got no brakes... I'd sure hate to have my family in the vehicle at that time...

Hope this helps. .

 

[Grinder1]

My 2002 Durango does this too. With engine running, push the brake pedal and it will continue to fall to floor very slowly.



However, I believe the proper way to test the master cylinder is with the engine off, trans in neutral. Push brake pedal a few times to relieve power assist, then it should come up and hold with light pressure. If the pedal holds, then your master cylinder is not internally leaking.



I have manually bled the system and finally broke down and bought a pressure bleeder (motive power) and it still feels "spongy". If you haven't bought a pressure bleeder yet, you are missing out. Its the best tool in my garage. Bleeding brakes is now a SIMPLE and FAST experience.

I sure do appreciate all the great info you guys are bringing up on this "falling pedal" topic, I find it very interesting that a 2002 Durango has the same symptom. My pedal is firm as a rock w/o power assist (engine off). Power bleeding sure couldn't hurt. What brand is yours and where did you get it?

 

[Grinder1]

Don't mean to get off topic but...





In a 95?? My 97 only has rear ABS.

It was optional in '95 and I thought it work out great on snow and ice, which it did. I no longer work the truck in those conditions and now really wish I didn't have the 4 wheel ABS. More expensive to repair and I now know way more about it than I ever really wanted to. OK now I've wondered off my own topic on boosters and falling pedals but hey this brake stuff is all related, right?

 

[BK]

my 95 GC (may it rest in peace) had the same issue , I read in many places it was common with anti lock brakes.

I'd have stopped, be sitting at the light, and the more I pressed on the brake peddle, the more it kept heading for the floor, slowly...

 

[Grinder1]

It's suggested that any brake system should have the brakes bleed once a year to cycle out the old fluid at the caliper and remove the water in the fluid. . We do this on our trucks. .



If you've had someone install pads on your vehicle, and do nothing but compress the piston, and install pads without opening the bleeder, you've pushed all that bad brake fluid back into the master cylinder... . you can later expect master cylinder failure...



This being said, when you can hold your foot on the brake pedal and it slowly drifts to the floor, it is a bad master cylinder and needs to be serviced or replaced... think about this. . how do you know that in a panic stop, and you put all your weight on that pedal that the seals (cups) in the master won't just give up and fail, and you've got no brakes... I'd sure hate to have my family in the vehicle at that time...



Hope this helps. .

Jim, thanks for your insights. Yes I have thought of the safety issue regarding this problem. I have done a whole bunch of emergency stops with my rig. Brakes work but I think they could be better. I think most everyone here would agree that the earlier Gen 2 trucks were under braked from the get-go which is why they introduced Hydro- Boost brakes in 1997. Keep in mind that my pedal does NOT fall during normal driving conditions at a stop. It is only when you start applying more pedal pressure when stopped that you would even notice it. I would ask anybody here to go out to your truck, fire it up, and gradually push down on the brake pedal. Let up a tad and push down even harder. What happens? I think some may have the problem and not even know it. This could be a generic problem. FYI if I'm not mistaken, most of the braking components on the early trucks are AC Delco

 

[Grinder1]

my 95 GC (may it rest in peace) had the same issue , I read in many places it was common with anti lock brakes.



I'd have stopped, be sitting at the light, and the more I pressed on the brake peddle, the more it kept heading for the floor, slowly...

Yes I'm exploring the possibility that it may be in the ABS hydraulic control unit. Dodge tech didn't think so. He thought it could be the booster which is how I started this topic. I think we should have a bunch of TDR guys go out and do a "pedal test". Trucks with and w/o 4WAL. With or without Hydro-boost. All different years. The results might be very enlightening indeed!

 

[B.G. Smith]

I still think it's the master cylinder. If you could get two screw in plugs to fit where the brake lines connect to the master cylinder you could eliminate that possibility. They do make them to fit most sizes. bg

 

[Joe Mc]

B. G. Has the right idea and ditto on the power bleeder if the mcyl checks out good.

Here's a link to one company's power bleeder - Bleeders I've been thinking on getting one for myself.

 

[ata]

It could be the Combination Valve also. The two lines that come out of the Master Cyl go into the combination valve. The combination valve keeps equal pressure between the front and rear brakes. If the front brakes fail to hold pressure the combination valve blocks the bad side so you will still have brakes on the rear. This valve usually goes bad when you compress the front calipers without opening the bleed screws when changing the pads. (debris)

 

[LRutigliano]

I sure do appreciate all the great info you guys are bringing up on this "falling pedal" topic, I find it very interesting that a 2002 Durango has the same symptom. My pedal is firm as a rock w/o power assist (engine off). Power bleeding sure couldn't hurt. What brand is yours and where did you get it?

TrackHaus sells brake bleeding tools and the Motive brake bleeder



Went out to the 2008 Chrysler T&C and with the vehicle stopped, trans in D and brake pedal pushed, you can get it to continue to fall with additional pressure.