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....

 

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.

 

Caliper replacement did not help.
Replaced all rubber lines with stainless steel flex lines, this did stiffen the pedal a little, but it will still slowly sink to the floor.
Made some plugs from brake line about 1 inch long, soldered the ends and plugged the master cylinder and the pedal is solid. Also tried plugging the master cylinder with the pedal half way down, thinking there might be a port or scratches letting fluid transfer from one piston to the other within the master cylinder, but it is solid at the 1/2 way point as well.
So I guess this proves the master cylinder and power brake booster are not the issue.
Has anyone ever experienced a similar brake issue, no fluid loss, but pedal slowly sinks to the floor BUT not the master cylinder????
Thanks!

 

It's an old thread . Did you get it fixed ? I have almost the same problems and did almost everything you did . I am a professional mechanic and it's driving me crazy. I found most parts are JUNK that's a big issue. My problem is LF brake skids especially first 3 stops, then much better the rest of day but still pulls left if I slam on brakes. Oreilly's stuff is really bad , I replaced 4 calipers in 1 month with leaks. also replaced their HydroBoost after 1 month, their pads in 6months and new disk in 6 months , it seems the pads are harder than the metal, which is Not cast iron it is Low carbon junk steel. I really feel the caliper is wrong on that side or the piston after researching different suppliers.
On your truck , I found that it makes a difference to disconnect the front lines at the master before bleeding the back breaks because the front brakes apply , and prevent the piston in master cylinder from full travel while bleeding the rear. I also used a 95 psi fuel pump to pressure bleed , from wheels back to master worked better than master down to wheels.

 

Unfortunately, I have not figured it out. I had to step away from it, as it was "getting under my skin". I do have a plan of attack once I get back to work on the truck. I plan to check pressure at each corner,I have a residual pressure valve I may install for the rear system and i also may potentially bypass the anti-locks if it becomes necessary.. I have a wilwood proportioning valve I may install to replace the factory valve. so, as you can see, I have given this alot of thought, just have not acted on it yet