Folks, I have a brake problem like I have never experienced before.

Problem: Brake pedal not firm, will go to the floor on first push. Brakes do work, will stop the vehicle, but the pedal goes to the floor. Even if you pump the brakes and get about half a pedal, it will sink to the floor if you push it down with the engine running.

96 Dodge Ram 2500
2WD, Rear Wheel Antilock
8800 GVWR
5.9 Diesel

A few weeks back the brakes on my truck felt mushy. I replaced both rear wheel cylinders and adjusted the shoes until they rubbed the drums. I have verified several times that no slack exists between shoes and drums.

While bleeding, I let the master cylinder run dry. Rookie mistake, but it happened. So I began to bleed the air out of the entire system. Initially I let the system gravity bleed until each cylinder / caliper had fluid. After that point, with a helper siting in the truck, I started with the lines at the master cylinder, then the lines at the proportioning valve, then the lines at the RWAL solenoid valve. From there I moved to the RR wheel cylinder, LR wheel cylinder, FR caliper and finally the FL caliper. All this was done carefully to not produce aeriated fluid. With no bubbles coming out anywhere, I have a pedal that will sink to the floor with the engine running. Without the power assist, the pedal feels fine. The truck will stop, but I don’t like a pedal that goes to the floor.

So I broke out the mighty vac, and vacuum bled the system at all 4 corners, pulled somewhere between 3 and 4 quarts through. I did initially seem to get some air, but with a vacuum bleeder it is hard to know if you are sucking air past the bleeder screw threads, even with them wrapped in Teflon tape. I tapped the lines and valves as I bled to dislodge air. I even removed the right hand side, hardline to rubber line bracket, so the upside down “U” bend was horizontal in case air was trapped there. No joy, still same pedal issue.

Next, I replaced the master cylinder with a reman. Bench bled it per the instructions with the plastic plugs. Installed it, bled the connections at the master cylinder. Same issue. Made some hardline bleeders from brake tubing and performed the conventional master cylinder bench bleeding making sure the lines were submerged in brake fluid. I did get a little air out. So I thought I had found the problem. Nope, same issue.

Bought another reman master cylinder. Same procedure, same issue.

Ok, so I am thinking air is entering the system somewhere. The calipers are old, I disassemble one of them and find light gouges in the piston. Thinking this is allowing air to enter as the piston retracts, I buy two reman calipers and install them along with new pads. Gravity bleed them, then with a helper, pressure bleed and vacuum bleed them. Same issue with the pedal.

I am hard headed, this feels like a master cylinder that is leaking internally .Back to the parts store and this (third) time buy a new master cylinder. Bench bleed it with my custom hardlines, install it. Same issue.

Next I try pressure bleeding from the bottom up. Buy more brake fluid, a new one gallon garden sprayer and attach my clear hose to the stalk of the sprayer and the other end to the bleeder screws. With a helper watching the master cylinder, I pump up the garden sprayer, prime it to remove all air from my new pressure bleeder and introduce clean brake fluid into the bleeder screws one at a time at each corner and fill the master cylinder 4 separate times with this method. No air bubbles appear in the master cylinder. Same brake pedal issue.

So to recap, up to this point, Rear wheel cylinders, Front calipers, Master cylinder and front Pads all replaced. All three rubber brake lines are one year old. Proportioning valve is centered. No evidence of leaks anywhere I can see. Brake shoes are slightly dragging against the drums and the front pads are in contact with the rotors. Pedal goes to the floor, although the truck has brakes that stop it ok, something is still wrong. Somewhere within the system a full stroke of the brake pedal is being absorbed.

Has anyone ever had this issue??? And how did you correct it?

Thanks! Not sure what to do next....

 

You have a leak in the system somewhere. The line running to the rear brakes likes to rust out, and break. Before it gets to that point, it may weep, or seep...... You won't always see brake fluid on the floor either..... it tends to pool on top of the tank, and drip off while you are driving.....

 

I will check on this and report back. Thank you for the information.

 

Checked the line running down the frame to the rear brakes, dry as a bone. Felt up inside the frame rail to see if there was any pooling, nope, just dusty dry. Applied the brakes around 20 times with the engine running so the power brake booster was helping, and held them tight. This was to try to up the pressure across the whole system to check for leaks. Found a very minor weep at the front right caliper, where the new washers were not seated. Tightened that up, bled that caliper, but no improvement. Still loosing pedal as I hold it tight. Everywhere else I could see was dry. I also marked the level in the master cylinder before the high pressure testing, it didn't budge. Where can a full piston stroke of brake fluid "hide" and return?

Thanks,

 

Pull the drums, check the wheel cylinders. (just to eliminate the possibility. doesn't matter if they are new.)

 

Thanks for the input. Yes I agree with that thought and I have checked both of them. No leaks either side of the backing plate. The pushrods are inserted in the cylinders and they are engaging both shoes. I even went as far as to pull the drum about 1/2 way off the shoes and have my helper apply the brakes so I could watch the shoes expand against the drums in case something was going on in there allowing lots of movement of the cylinder pistons. Almost imperceptible movement as the shoes were adjusted to slightly drag against the drums.

The one thing I did notice from this exercise, was that the rear brakes don't apply until the pedal has moved a very considerable amount. As I have never examined rear shoes in this manner before, I don't know if this is a symptom of my issue or if it is normal.

I also grabbed each rubber line to see if they were ballooning as the brakes are applied. They just stiffen up a little. However I have ordered stainless Russell flex lines, part number 694500 as I know they will flex less than new rubber lines. I don't think they are the problem, but I do like the feel of brakes with stainless flex lines.

 

Hhhmmmm...... Wonder if you still have some air trapped in the rear line......

For stubborn bubbles..... I have my partner in crime pump the brakes up, and hold pressure, while I crack open a bleeder. Do that a couple times, and it will usually shock loose any stubborn bubbles...... But, the fact that you pedal slowly sinks to the floor, suggests that pressure is bleeding off somewhere........ Question becomes, where?? Might unbolt the master cylinder from the booster, and pull it out a bit, and see if there is an internal leak there. (yeah, I know, it's new..... check it anyway.)

 

Time to plug either the front or the rear line port on the master it self. Split the system and diagnose from there maybe

 

I've checked a number of things, fabbed up some test connections and replaced some stuff:
Checked the Master cylinder to see if it was leaking out the rear into the power brake booster. Nope, dry.
I did notice that the power brake booster pushrod/piston and seal were dry and dirty. So I cleaned both and applied silicone grease. Hopefully this will address a typical lazy power assist I have at first cranking each morning.
Made plugs from brake tubing for the front and rear ports of master cylinder and for the tee-fitting for the crossover tube at the front.
Checked pressure at master cylinder.
Front portion (rear brakes) easy to obtain and hold 400 to 600 psi with a plug in rear portion (front brakes) of the master cylinder, this is without power assist.
Rear portion (front brakes) as soon as 400 psi is obtained, the pressure immediately starts going away. Tried additional bleeding, both pressure and looped line back into reservoir to no avail. Therefore master cylinder (yes it is new) is bad. Replaced master cylinder and with plugs in it, have rock solid pedal and steady pressure at each port.
So, I now have one item, master cylinder that checks good.
Moved down to the proportioning valve and discovered it is now leaking out it's sensor port. Don't know if it was leaking when all this trouble started, I had bled it a few days back by removing the large nut, and that may have damaged it's internal seals. Went to the pick a part salvage yard and found a 3/4 ton, V-10 truck, same year as my cummins powered truck, with what visually looks to be the same RWAL solenoid block and proportioning valve. Checked the resistance of both solenoids, one was 1.7 ohms and the other was between 4 and 5 ohms, just like my original solenoids. Installed both the proportioning valve and RWAL solenoid block as an assembly. Plugged the outlet of the proportioning valve to the front brakes, bled the air out of master, prop valve, RWAL. Rear brakes are rock solid pedal. Yeh! I have isolated the issue to the front brakes.
Hooked the front brake line back to the proportioning valve and bled the air out at this fitting.
Plugged the tee fitting at the top of the rubber hose on the driver side to remove the crossover tube and passenger front brake from the system. Obtained a good pedal. So now the problem is either the crossover tube or the passenger side brakes.
Hooked the crossover tube back up to the driver side tee fitting at the rubber hose.
Went to the passenger side front wheel well and unhooked the crossover tube from the rubber hose. Took an old rubber hose for that side and cut off the hose from the brass end, cleaned and tinned and soldered up the fitting to have a way to pressurize the crossover tube. Then realized my pressure gage would screw right into the crossover tube. So I installed the pressure gage instead. Bled the air. Had my helper stab the pedal. 1600 psi! and steady! This was with the engine running and power assist.
Hooked the crossover tube back up to the passenger side rubber hose.
My new metric tap set arrived today, good timing. Took a piece of aluminum round stock and drilled and tapped it for M10x1.5 to attach the end of the rubber hose to it as if it were the caliper. Bled this joint. Good pedal to this point. Therefore the passenger side caliper is suspect.
Disassembled the caliper and found multiple impact marks across the sealing surface. It is obvious that this is a new chrome plated piston in this newly installed reman caliper, but someone was very rough with it. Removed the square cut seal and found steps in the bottom of the seal groove where maybe the caliper had a repair attempt during reman? Anyway I suspect the impact marks &/or step is allowing air to enter during retract. There was no evidence of a pressure leak, so it must be drawing in air. That is my theory at this point.
Tomorrow I will replace the caliper and see.

Thanks folks for the helpful input on this issue!

 

Well, that was thorough. Let us know if the caliper replacement solves the issue.