2nd Gen Non-Engine/Transmission Falling brake pedal, need ideas

[SMorneau]

My 96, with 4WABS :{ is giving me brake fun again. This time it's got new lines, SST hoses, rotors, calipers, cylinders, etc. as part of a kind of reconditioning process All of this post is in reference it static testing in the garage; no road testing or driving yet since the upgrades.



The symptoms are described in several threads on this site but it doesn't seem like anyone followed up with what actually fixed it . I get a good firm petal on the intial push and then the pedal just falls to the floor under any foot pressure at all. You can pump it and the process just repeats. It only does this while it is running, I assume because the booster applies much more force than my leg.



The prescriptions in the threads generally go as follows: Master Cylinder, brake booster, RWAL valve, leaks, and too much air in the system.



The system was pressure bled and I don't believe there is any air in it. Even if there was, that would make for a spongy pedal but not falling away.



The master cylinder sounded like the problem to me. But rather than start throwing $130 parts at the problem I made some master cylinder plugs (directions on making the plugs are below) for the ports on the master cylinder and with both ports plugged the pedal is hard as a rock no matter how long I hold pressure on the pedal; no falling away. With just the rear port (services the front brakes/ ABS snafu) blocked the falling pedal is there nice and solid with no falling. With just the front port (rear brakes) blocked the pedal falls away.



I interpret those results to mean: 1) the master cylinder is good and should not be replaced with a new mastercylinder, especially a rebuild which could be worse than the original. 2) The rear brake circuit is good because with it connected the pedal was nice, 3) the front circuit is loosing pressure somewhere. Since there are no leaks and the fluid is incompressible it must be accumulating somewhere. I suspect the ABS is leaking to an internal accumulator, even though it has been electrically disconnected for several years.



The brake booster seems to work perfectly as the pedal performs well and I don't know why that would make it fall the vacuum is built up when I press the pedal initially. I reconnected the ABS fuse and both the red "BRAKE" light and the yellow "ABS" light behave normally. No change with ABS connected.



At this point I am looking at buying flaring tool and splicing the lines around the ABS unit, leaving the RWAL valve intact physically but not electrically.



Anyone else had this problem? What am I missing or does it sound like I'm on the right track?



Scott





MASTER CYLINDER PORT PLUG INSTRUCTIONS

To make port plugs for the 19 4x4 master cylinder go to Napa and buy one 813-1156 adaptor and one 813-1157 adaptor. You can cut the tube about an inch or two away from the tube nuts that fit the master cylinder ports, put some solder in them, give them a crimp, and then melt the solder from the outside while holding the flared ends up. These would also be good to make bleed lines that loop back to the reservoir, like they show the manual. They cost about $3 each.

 

[TBurow]

still sounds like a master, could have a worn spot partway down the bore that lets fluid past the seals internally, if you plug it you are not getting any pedal travel and not seeing the worn part of the master. seen it many times, that is why it holds if you pump it up and hold it, you pressurise the system enough so the pedal doesn't have to go down so far. here's a test un adjust the rear shoes some and plug off the line for fronts, recheck the pedal now that it drops moree due to rears out of adjustment if it does you need a master. if not try un adjustind some more if still good bypass the abs unit. hope this helps.

 

[SMorneau]

Good point. I ordered a MC from Rockauto. I'll update when done. I'd like to get the abs unit out of the way but I would be happy if the MC fixes the falling pedal.



thanks,

Scott

 

[SMorneau]

I installed the new Raybestos MC and still have falling pedal



So I bypassed the 4WABS, which had some pretty rusty lines going to the front wheels anyway, pressure bled and still have falling pedal.



So I bypassed the RWAL valve as well, bled again, and still have falling pedal



So with the new master cylinder piped right to the combination valve and then to the brake calipers and cylinders the pedal still drops away.



So I plugged off the rear port on the combination cylinder, which serviced the rear wheels, did not bleed, and the pedal falls.



This one has me thinking I am not going to miss the ABS since it doesn't work well anyway and the lines were in poor shape. I am very happy with the lines I assembled with adaptors, unions, and tees it looks good and isn't leaking so all is not a total loss. The pedal is a seems like it might be a little better but it's hard to say.



So I figure the next step is to get another 3/16 plug and plug off the front port on the combination valve and see what that does. Can anyone tell me exactly what the combination valve does? This thing has me baffled and I don't see how a caliper(s) (rebuilds from Advance Auto)could do this and I don't want to shell out for a new combination valve if I am not pretty sure it's bad. I also do not want to bypass it because it indicates a lost brake circuit and it takes 1/4 connection on one side and a 3/16 on the other so I don't even know how I would bypass it.





My brake hoses are new stainless steel and I am not finding any leaks anywhere. With all of the pedal pumping I am doing there would be a pretty big hard to miss mess somewhere.





I am pressure bleeding at about 8 psi and I have good flows to all the rear wheel cylinders and a little less flow to the front calipers. I have bleed out about 200 ml at the right rear and got no bubbles from it after about 80 ml. Repeated bleeding at each wheel and usually observed a small group of bubbles then solid fluid. Seems good to me???



Help!

 

[TBurow]

ok, just have to ask if the calipers are on the proper sides? have seen several professional tech's put them on with bleeder facing down, not saying you did but doesnt hurt to ask, next are the pads in front binding on the front hardware and working like a spring pushing the caliper piston back in when you let off the pedal?



the combination valve is also called a proportioning valve and controls pressure bias between front and rear. usually when theese go bad they start to leak externally, however sometimes they stick inside and can trap air bubbles inside, while you are pressure bleeding try tapping on the valve with a wrench to move the bubbles around and hopefully out of the system.



as far as by-passing the valve they sell 3/16 to 1/4 adaptors but they are hard to get ahold of sometimes, just check at autozone or advance or any large auto parts store. but if you disconnect it you may have problems with wheel lockup during hard stops. hope this helps

 

[SMorneau]

The calipers are on with the bleeders pointed up, which is the way they have always been.



I will plug the front brake circuit coming out of the combination valve and see if the pedal stays firm. If is does I will pull the calipers and clamp the pistons using some C-clamps to check your other theory. I am also going to tap the combination valve and the calipers when I bleed them.



I am also going to have someone hold the brake pedal down halfway while I power bleed the fronts again. Could be there is some air in the system yet. I am speculating the booster keeps building force and compressing the air that is in the system. I notice the flow is somewhat less when pressure bleeding the front then it is in the rear. Maybe the flow is too slow to carry the air out of the system.





I don't want to bypass the combo valve permanently, espescially if it is a proportioning valve, but I might give it a go for troubleshooting. What does the adaptor look like? It would have to join a 3/16 tube flare to a 1/4 tube flare (not just the tube nut sizes but the tubes are different). The other adaptors I have used are short pieces of line with different size tube nuts. Would these adaptors look like a union?



I appreciate your posting and buy some stock in the brake fluid companies my usage is way up



Scott

 

[TBurow]

the adaptors are small brass fittings that have 3/16 female or male on one side and 1/4 male or female on the other depending on what one you choose. all they do is step up or down the line size you will still have to theread them into a unuion just like connecting two steel lines with same size nuts.

 

[jelag]

A drifting brake pedal can be caused by one of 2 things... .

1 - a leak... .

2 - master cylinder... .

the abs would have to leak to allow the pedal to drift to the floor... . its can turn on and off the brakes, but has no place to store a large amount of brake fluid that comes from a pedal going to the floor... .

If you don't have a leak..... and the calipers are installed correctly with the bleeders up... so there is no air in the system it has to be the Master Cylinder... .

Did you install a new one or rebuilt one... . if a master cylinder is not assembled correctly the ports in the cylinder will not be covered or exposed at the correct time and can cause this problem... .

I would not personally drive a truck with a modified ABS system... I'd put it back the way it belongs, and question the master cylinder you just installed...

Pull the lines from the master cylinder and plug them... ... pump it up and let us know what's happening...

 

[SMorneau]

The master cylinder is new and when plugged does not allow any pedal drop.



The ABS was crap from day one and it almost caused me to have two accidents before I pulled the fuse a few years back. It simply does not work on snowy roads. I will never put it back! Ever! It was a bad implementation of a good idea.



I bled the system again last night and did not get the pedal fade but I could not get a nice solid pedal either. When I reconnected the rear brakes at the new union directly after the combination valve the pedal fade/drop came back.



I am going to bleed the system again and then wait until I get some drive time on it to see how it does. The only two things left in the sytem are the booster and combination valve.

 

[SMorneau]

New booster, bled again, same old *****!



Today I blocked off the steel lines that connect to the new Stainless brake hoses and I have the kind of pedal I am looking for...



So it looks to me like the rebuild caliper are crap. I was told that some of the rebuilder are spinning done the old pistons to clean them up. So that makes them smaller and removed any plating that is there to harden them up.



So I'm off to find some calipers. EGR? Maybe.



I should have never started throwing parts at this problem it rarely provided a me with any satisfaction; even when if fixes the problem.



I will update when I get working brakes.



Scott

 

[Hoefler]

How far is the pedal falling?? Does it go a little way and stop or does it go to the floor??

 

[SMorneau]

when I block off the hard lines at the ends that go to the hoses on the front calipers I got a nice, hard, high pedal that does not fall away. That leaves the new stainless hoses or the A1Cardone rebuilt calipers. I will block off the hoses leaving only the calipes out of the system to see what happens. I will update when I get it done.



Scott

 

[SMorneau]

So after replacing the calipers with some EGR calipers the system behaves the exact same way. Since my last post I also bypassed the combination valve and it made no difference. I reconnected the combination valves since I think it is necessary for the proportioning and "hold off" functions. I also installed another adjustable proportioning valve in line with the rear brakes directy after the proportioning valve.



I also replaced the stainless brakelines with standard hose. I don't want to start anything here but some reading indicated there was little to no advantage to them and they don't have the established safety record of the standard hoses. Certainly in the case of my brakes a little hose flex isn't going to make much difference. Read on.



So with new calipers, hoses, master cylinder, etc. My friend Kevin and I bled the system, starting with the master cylinder FSM method, and the pedal method on the rest of the system and the system behaves the same. So we hooked a vacuum gauge ahead of the booster and as the vacuum pump builds vacuum you can trace the pedal movement to the floor while it is under med foot pressure. I know, I know, it sounds like air but I can't imagine where it would be.



Anyway I have had plenty of time to think about this pedal behavior and I guess I can't be certain the pedal ever held under constant foot pressure. It is absolutely something I could have never noticed. With the new additional proportioning valve I could dial the rear drums back so they were out of the picture (not locking up under med to hard braking) and get the new calipers and pads settled in. After about 15 minutes of breaking them in and adjusting the rear brakes back into the picture they all began to actually stop the truck. I am going to continue to use the truck (this truck was not driven much the last 2 1/2 years or so) and see if they continue to get better. Kevin who has a CTD thinks they are working and I'm used to driving a Jetta so what do I know.



Here are some closing thoughts:

1) I think the adjustable proportioning valve will work nicely to severely reduce rear wheel lock up. I will give it full strength to the rears when I have a load. I think I will highly regard this modification as time goes on.

2) While I don't feel good about throwing money at a problem it is the brake system and I at least know the system harware is new and reliable.

3) I now know a lot more about brake systems including metal brake lines.



I will update if anything changes but for now my brakes seem to be working well, with the exception of the pedal behavior that really doesn't seem to play into everyday driving anyway. No pulling or darting on the hard stops and the rears are dialed back not to lock. Time will tell.



Scott