hmmm, now ya got me thinking about these oversize wheel cyls. i have the
1-1/8" cyls from napa, the part number is listed on this site as an acceptable upgrade.

i can see how the larger bore would have an effect on my pedal travel. hmmm, well i guessi can try the 1" wheel cyls, but i am kinda getting tired of taking the drums on and off.

do 250s and 350s use the same MC? wait, i answered my own question, they do. ok, i'm done now

 

Problem is, they work for other folks! So even though they may be a part of your problem they're probably not the whole problem. Seems like it could be a bogus master or power booster, and the oversize wheel cylinders and maybe some weak lines magnify the problem. I'd still look at the master first, even swapping on a used one or whatever, then go from there. I have no idea how to 'test' a brake booster 'cause all my trucks and cars have always been manual brakes.

Hey, I do have a '90 Dodge Truck Factory Service Manual, so let me see if it gives a test procedure on master and booster.

 

Brake Test Procedures:

Pedal falls away under steady pressure: generally the fault of system leak: could be brakeline, fitting, hose, caliper or internal leak in master.
Low Pedal: pump several times. If pedal comes back up worn linings, rotors or drums are likely the problem. [Note: oversize cylinders could contribute to low pedal]
Spongy Pedal: most often caused by by air in system. Thin brake drums or substandard lines or hoses can be another cause. Bleed and if necessary replace thin drums and suspect lines. [Note: in your case the oversize wheel cylinders could contribute to a spongy pedal]
Hard Pedal: may be caused by wet, glazed or contaminated lining. Power Booster or Booster Check Valve are another possible cause.
Fade: caused by overheating from brake drag or faulty linings.
Pedal Pulsation: caused by worn or loose components, faulty rotors, drums out of round, loose calipers or bearings or worn/damaged tires.

Master/Power Booster Testing: With engine running check booster's vacuum connection and also listen for hissing. Shut engine off, place gear selector in neutral and pump pedal until all reserve vacuum in booster is depleted. Press and lightly hold brake pedal: if pedal doesn't hold pressure and falls away master cylinder is leaking internally. If pedal maintains pressure, start engine in neutral and if pedal falls away slightly then holds firm proceed to Booster Test Procedure. If no pedal action is noticed the Power Booster or Vacuum Check Valve is at fault. Install good check valve and retest.

Booster Test Procedure: Restore booster pressure by releasing brake pedal and operating engine at about 1,500 rpms, release throttle and immediately turn off engine. Wait at least 90 seconds and try brakes again. Reserve vacuum in booster should provide at least two or more vacuum assisted brake operations. If not, perform Booster & Check Valve Test.

Booster & Check Valve Test: Disconnect vacuum hose at check valve, remove check valve and seal from booster. A hand operated vacuum pump may be used to apply 15-20 inches of vacuum [50-67 kPA] at hose end of Check Valve. Vacuum should hold steadily and if pump gauge shows loss of pressure, Check Valve is at fault. If Check Valve tests properly reconnect Check Valve to Booster fitting. Connect a Vacuum Gauge to Check Valve with a length of hose and a tee fitting, with tee fitting installed between Check Valve and Manifold Vacuum source. Start the engine and run at idle for one minute then clamp hose shut between Manifold Vacuum and Check Valve. Stop engine and observe vacuum gauge reading. If reading drops more than 1 inch of vacuum within 15 seconds either the Booster Vacuum Diaphragm or Check Valve are at fault.

JimmieD

 

Have you bypassed the RWAL dump valve?

 

wannadiesel- I'd say it is.

Also, on rear brake adjustment- the manual says to adjust it to a slight drag, then (assuming new or at least good adjusters) do several reverse stops, coming to a hard and COMPLETE stop, or something to that effect. It wont be your entire problem, but it is likely a contributing factor, if you adjust to slight drag then leave it. Combine that with a bad MC, and bench bleeding issues, and you could hav eyour problem.

Many new masters come with the plastic plugs to bench bleed it with, but I imagine the brass, etal, plugs would do just as well. Are you letting fluid dribble out before starting your plunging? I've been told it can score the seals, and cause internal leaking if you pump it dry.

Daniel

 

Just a thought... I believe the booster output rod on our trucks is adjustable. You can measure the distance from the output rod to the mounting surface, and from the mounting surface on the master to the bottom of the piston and calculate the gap. It should be close to zero. You can't make an exact measurement with out some pretty fancy jigs because the booster diminsion will change a bit when vacuum is applied, but you can get an idea. From there you can adjust the rod in the booster. You might have to make a tool to hold the outside of the booster rod so that you can turn the adjuster. If the gap is very large it will contribute to a low pedal. You want the gap to be as tight as possible, but the master cylinder piston needs to be able to return far enough to uncover the fill ports and not trap pressure in the rest of the system. Clear as mud?

 

jimmy d- thanks for that useful info from the tech manual.
daniel-appreciate all the info, you seem up on this stuff.
when you bleed your master, you first plug the outlet ports, pump it until it gets hard, then unplug the holes, and run lines up into the reservior and repeat?

i went out yesterday and got one of those "mityvac" vacuum bleeder things w/ the vacuum guage so i can at least check the system out.
i am also gonna try the whole back up and slam on the brakes routine, havent done that yet. how fast does one go in reverse before stopping?

what is the point of the coiled up brake lines that come out of the MC, and what is that "block" with the wire attached to it?

 

Nobody said anything about "slam on the brakes." Just a couple of good firm stops going in reverse at about 10mph should do the trick. Don't be leaving skid marks. Not sure what the "coiled up brake lines" means, but the proportioning valve with a pressure sensor ("wire attached") is the block I believe you are referring to.

 

What Daniel said about plugging master ports and bleeding in short strokes can be very important. I had one master, not on my '90 but another truck, that could not be bled by normal methods of full strokes on pushrod. The ONLY way the blasted thing would bleed is with 1/16"-1/8" strokes, then it worked fine. Changed master again a few years later and that new one bled out just fine with full strokes. Strange...

Your best bet now is step by step troubleshooting, diagnosing each component one by one in a process of elimination. Even if you find a problem go ahead and fisnish the procedure in case it's several issues going on at once.

 

you dont have brake lines that are in a coiled roll, (think coiled like a coil spring) right under your mc? i just wondered what the point of the coiling is.




see them there, under the pressure sensor, next to the steering shaft? (not my truck, but the pic illustrates my ?) i dont see them in every truck, just wondered the point of it.

 

Those are 'surge coils' and they serve 2 purposes.

One, they prevent a fast surge of fluid to wheel cylinders which can cause brakes to grab. Fluid or air loses velocity in a bend, and the loops slow down the speed of motion of the fluid while maintaining the same volume moved.

Second, they act as anti-fatigue springs, preventing vibrations in the vehicle from causing metal fatigue i.e. broken brake lines. 3rd, they look trick ha hah!

Hope things work out for you this time. What a royal pain brakes can be.

 

coiled brake lines are also coiled to help cool th brake fluid.... well thats nisan's heory anyway...