2nd Gen Non-Engine/Transmission Brake pedal to floor intermitently

[Trophy Truck]

Gentleman, Please help. I replaced the front brake pads on 96' 2500 Diesel, 4x4. After bleeding the brakes and using new brake fluid, the brake pedal goes to the floor INTERMITENTLY when you depress it with mild pressure. If you pump them up, good pedal returns (with engine running, bled from farthest to closest calipers to the master cylinder... . I replaced the master cylinder with (2) rebuilt ones and a new one. Even replaced the line from the front of the master cylinder to the ABS unit. Still, the exact problem. Flushed the brake system after bench bleeding the master cylinder. Someone told me that the ABS has to be actuated (by skidding the tires), but this did not work. Any words of wisdom would be appreciated. Thanks.

 

[budnate]

sounds like the master cyl is bad,



B.

 

[Skydiver]

Sounds like a bad master cylinder. Could still be some air in the line.

Are you making sure the master cyl resevoir is not going dry while bleeding?

 

[GCroyle]

I just replaced my MC on my '97, I claim no expertise.



Q. Has the same intermittent problem been with all MC replacements?



If it has repeated, then I'm think more along the line of a trapped air bubble.



I had real good luck bleeding my MC R&R before pressing on the brake pedal by vacuum bleeding the last bubbles out with a hand held vacuum pump going directly through the fluid into the port, I could see the air bubbles coming out in the clear vacuum line. If the bubbles air in the bowels of the system, then full line bleeding sounds necessary.



Good luck.

 

[SSolderitch]

Did you bench bleed the master cylinder before the install?

 

[B.G. Smith]

Are the bleeders at the top of the caliper? bg

 

[DonTX]

Air does not come and go, INTERMITTENTLY. The only way this described thing can happen is either the fluid us getting past the MC piston, or it is leaking out somewheres. No indication was reported of a leak.

Try this once: Push the pedal down VERY, VERY SLOWLY, and see if it goes to the floor. It did? Ok, now hit the pedal with all the foot force you can muster for a QUICK pedal travel, did not go to the floor? Gotta be the MC piston rubbers.

Gentleman, Please help. I replaced the front brake pads on 96' 2500 Diesel, 4x4. After bleeding the brakes and using new brake fluid, the brake pedal goes to the floor INTERMITENTLY when you depress it with mild pressure. If you pump them up, good pedal returns (with engine running, bled from farthest to closest calipers to the master cylinder... . I replaced the master cylinder with (2) rebuilt ones and a new one. Even replaced the line from the front of the master cylinder to the ABS unit. Still, the exact problem. Flushed the brake system after bench bleeding the master cylinder. Someone told me that the ABS has to be actuated (by skidding the tires), but this did not work. Any words of wisdom would be appreciated. Thanks.

 

[GCroyle]

Don/TX



I tend to agree with your summary, but if he has reported the same symptoms with 3 MC's how do we explain that?



On my '97 the brake light and ABS light occasionally lit up and I felt the pedal un-explanedly go down too far, then the next stroke it was OK. My Son reported same pedal mystery. I took the old one apart, didn't see the culprit in the old MC.



I did bench bleed my new MC and then quickly attached lines and then used the vacuum pump to pull out bubbles in the line to MC connection area, new MC has been fine for a month now.



I wish we had a litmus test for the presence of air in a hydraulic system but I think I'm dreaming. I know it has befuddled the clutch industry for decades and ain't going away.



What about a failure or pending failure of a rubber section of his brake lines?



Why say you TDR brake guru's?

 

[GAmes]

The 96 is vacuum operated I believe, not like a 97. I have read that multiple applications too close together can cause the brakes to not function correctly.

 

[fest3er]

I'll throw in my tuppence and mention the other parts of brake system heretofore unmentioned.



First, I'd like to verify that the pedal goes all the way to the floor (or to whatever mechanical stop there is), just to be sure we haven't a Cool Hand Luke here (failure to communicate).



So, there's the proportioning valve that can cause the brake pedal to drop and loss of braking of it ain't workin' right.



Next there's the ABS (RW, AW) if the truck was so equipped; if it ain't working right, it might cause excessive pedal travel.



Then there's the possibility that some anomaly in the front or rear friction system is causing pads/shoes to be pushed away from the rotors/drums, thus causing fluid to be pushed back into the reservoir and causing lots of pedal travel to get the pads/shoes back where they need to be to create friction.



As mentioned, there's the master cylinder. But I've never encountered a fresh (new or rebuilt) M/C to fail. And certainly not several in a row.



Then there're the wheel cylinders. Leaking? There'd be evidence, and the rear brakes might be especially grabby.



If there was air in the system, the brakes would be spongy, not intermittent.



As Don/Tx said, if easy pressure (like almost none) lets the pedal sink to the stop ever so slowly while a sharp strike keeps it solid, it'd most likely be the M/C. I had a '66 Gutless that did this with its single M/C. Light pressure it would ease to the floor. Rapid pressure and hold it, the brakes would hold, at least until I eased off the pressure, whence the pedal would again sink to the floor. (It was like that for so long, I actually missed playing with it at stop lights after I fixed it!)



Sure sounds like the M/C, but I find it hard to believe that *both* cylinders have failed the same way; *one* of them ought to keep the pedal from going to the floor. TT, did you completely flush the old fluid (front and rear)? What did the old stuff look like? (Opaque? Rust-colored? Nearly clear?)



And thanks for reminding me. I think it's nigh on time for me to change my brake fluid again (every 60-100K miles).

 

[DonTX]

Don/TX



I tend to agree with your summary, but if he has reported the same symptoms with 3 MC's how do we explain that?



Well, of course we can't explain it! Still physics are physics, if the pedal goes to the floor, and the fluid has not escaped, it had to get by the piston or squirt on the floor. The option that some shoes or pads are backing off contact for some reason could hold water, but that would overfill the MC and leak if it were happening, and something would have to back them off (maybe rocking the truck, driving it some, whatever). Some hint of when it does it and when it don't would be helpful. .

 

[surfbeetle]

I would say that the master cylinder has not been bled all to the way each time before bleeding the entire system. When bleeding, you will need to start with the right rear, then left rear, right front, then left front. I used a harbor freight air power bleeder. I had the same symptoms before using the power bleeder when I changed my master cylinder.

 

[Trophy Truck]

Stomped

Well, I am still stomped, I looked at all the great suggestions that you guys gave and checked all of them out. To review, I have encountered the problem as follows: Excuss my verbiage, it late.



*Replaced front pads only (All break lines are good - even the rubber hoses up front, no leaks anywhere in the system as far as I can tell). Removed brake lines at the caliper on both sides of the front end, bled brake like I aways do.

*When I took the truck out for a test spin I was sitting at a stop sign with moderate pressure on the brake pedal and it slowly went almost all the way to the floor, a couple of pumps and normal pressure was back, a few more miles of driving and the symtom returned.

*Completely drained the break system (the color was dark and full of debris that was caught in my brake fluid catch can

*installed rebuilt MC filled with Dot 3 Prestone synthetic flluid after bench bleeding with the retainers screwed into the MC outports

*rebled the system from farthest wheel back to the closest to the MC, did this 4 times with engine running

*encountered the same problem with pedal going to floor INTERMITENTLY

*Went through the same drill with 2 more MC

*Actuated the ABS system by attempting to lock up the brakes and feeling the brake pedal pulse

*rebled the system with even more fluid, so this equates to about 4 quarts being flushed through the system, again, no leaks and no more or less brake fluid abnormally in the MC during the whole event.

*Skydiver note - the MC is not going dry when bleeding

*GCroyle note - I might have air still in the line, you never know, I will try to locate a vacuum pump

*SSolderith note - the MC's were all bench bled with the caps in place prior to installation

*BGSmith note - the bleeders are on top of all the calipers

*DonTX note - After 20 to 30 brake depressions driving or bleeding, the brake pedal was pushed to the floor with moderate pressure similiar to brake pressure when stopping, no leaks or additional fluid in the resourvor. I agree with the MC rubbers, but after 3 MC's?

*fast3er note - I will have to look more closely at the proportioning valve, I remember long ago that I had to hold out the proportioning valves out while bleeding a vehicle about 20 years ago, I may try that next. I will also pull the rear brake drums to see if the brakes are not expanding abnormally and pushing fluid back in the MC, but no additional fluid noticed in the mc.



Thanks,

Pete

831-638-0194

 

[GCroyle]

Pete,



Good luck solving this one. I know we are all interested in what the final solution or source of the problem is.



There are many ways to bleed a master cylinder actuated hydraulic system (I need to add clutch here) and from my efforts, I would reach for a hand vacuum pump as a serious tool. As I stated, I finished vacuum bleeding my MC through the fluid, bit of skill bigger bit of luck. But where vacuum bleeding can shine is in its ability to pull large quantities of fluid in one operation, cleanly and you get to see results in the fluid flow, like debris or bubbles.



My home brew rig consists of a hand vacuum pump and as large of a catch bottle as you can rig up, quart is small, 1/2 gallon can be powerful. I rigged a rubber stopper in the catch bottle with 2 holes, inserted short sections of 1/4" copper tubing, one goes to vacuum, one has a tubing extension that allows the incoming waste fluid to go to the bottom of the reservoir.



Prior to attaching the line to the bleed screw, I remove the bleed screw and apply teflon tape to the threads only, not the seat area. This is to make as effective of a seal as possible for the vacuum to pull fluid, not air around the threads. It may take a few minutes of pumping to build up the necessary 15-20" of vacuum, once you get that, open the bleed screw and have someone ready to add fresh fluid as the vacuum pulls out a lot of fluid, flushing and purging. Continue hand pumping and topping off, watch fluid, cross fingers.



I do not know what your exact problem is, I just play with clutches and their related use of DOT 3.

 

[DonTX]

Where did you get all those MC? Rebuilt or new? Is it possible that a reuilder got a batch of them assembled with wrong size rubbers?

The dark color with debris fluid is noteworthy, what do you attribute that to? Have you taken any of the MC apart?

I have bled brakes on motorcycles and aircraft that are total beeches to bleed, but the air never caused the problem you are having.

 

[B.G. Smith]

I know you are wondering why I asked what position the bleeder screws were in, probably thought just another crazie. I once was called by an aquaintence to go to a repair shop with him to see if I could determine why the mechanic could not get the brakes bled on his Dodge truck, they had had it for two weeks. The mechanic had installed new calipers but had switched sides with them, bleeders on bottom. It's kinda hard to get the air out the bottom. bg

 

[fest3er]

I know you are wondering why I asked what position the bleeder screws were in, probably thought just another crazie. I once was called by an acquaintance to go to a repair shop with him to see if I could determine why the mechanic could not get the brakes bled on his Dodge truck, they had had it for two weeks. The mechanic had installed new calipers but had switched sides with them, bleeders on bottom. It's kinda hard to get the air out the bottom. bg

I hope you charged the shop $20 for the on-site call and $2,000 for knowing where to put the X... .

 

[DonTX]

I know you are wondering why I asked what position the bleeder screws were in, probably thought just another crazie. I once was called by an aquaintence to go to a repair shop with him to see if I could determine why the mechanic could not get the brakes bled on his Dodge truck, they had had it for two weeks. The mechanic had installed new calipers but had switched sides with them, bleeders on bottom. It's kinda hard to get the air out the bottom. bg

I bought one like that once, the comedy was that I supposed that the calipers were somehow messed up, so I pulled the calipers, turned the bleeder up to the top and bled them, then reinstalled them with bleeder down. .

About a week later it hit me, the darned things were on the wrong sides!! DUH!