Loose brake pedal

[Lviz]

Hi all, got a 93 D250. I’ve read around various forums about the RWAL valve and people bypassing it and keeping. I have also read a lot of forums of the Dana 70 disc brake conversion. Now my problem, changed the brakes all around, front calipers, pads and flex hoses. Changed the rears to disc, and bled the system as much as I could. Has anyone ever experienced the pedal going to the floor, no resistance at all, brakes don’t grab. Any help or advice is greatly appreciated.

 

[dieselshadow]

You either have a huge air bubble in the system or your master cylinder is done.

When you bleed the brakes, does it pump fluid ok? Are you getting good clean, air free fluid? Did your helper let the master cylinder get too low?

What did you do for proportioning front to rear now that you have discs in the rear?

 

[Lviz]

I was getting a decent flow of clean fluid bubble free. On one occasion after doing the rear, checked the master and it was low, so re-bled the system. As for the proportioning I haven’t messed with that. I’ve spent two days bleeding it, with the truck on and off. First time around, when off, got the pedal to feel firm. Turned it on to try it out and pedal went straight to the floor.

 

[B.G. Smith]

This is embarrassing question but stranger things have happened. You didn't swap sides with the calipers did you?

 

[Lviz]

Didn’t even think that was possible. But no they are on the correct sides.

 

[B.G. Smith]

Yes, I had a friend who had his in the shop for a week for new front calipers/brakes and they couldn't get a pedal. They finally discovered the calipers were switched which caused the bleeders to be on bottom.

 

[Lviz]

All the calipers are in correct position. I guess my biggest question is if anyone has had this problem with the disc brake conversion that still has that RWAL valve still installed.

 

[Lviz]

Edit- message uploaded twice

 

[RWherley]

might be a silly question but is your fluid level in the master holding steady or is it going down?

 

[Capt. Methane]

A little history lesson...

There are a couple of advantages to disk brakes and the first set I worked on I thought "What a great invention!" but those were actually Cleveland brakes on a 1963 Cessna 150, they are much the same as automotive "disc brakes" though they've been on light aircraft since the late 1950's.

First of all, those are not "Disc Brakes" by technically correct definition. They are Puck Brakes. There are true disc brakes on larger, faster aircraft and road tractors where multiple pistons push on a stack of alternating steel or carbon plates and brake material in discs that are concentric with the axle and inside the wheel.

Consumers have been sold a bill of goods regarding so-called disc brakes in newly manufactured cars and the after-market world and here's how...

Presuming that the quality of engineering and production are equal-

The puck type brakes are lighter because they use less materials, have far fewer parts, are far simpler to manufacture and work on, all while wearing out faster than a comparable drum brake due to being completely open to the world of dirt and grit and having less material.

Thus initial production costs are reduced and downline maintenance frequency is increased while profits are enhanced by allowing the same rate to be charged per axle for brake repairs!

The only performance enhancements come from the reduced mass of the puck brake system and open design which of course does allow for more rapid cooling. In motor sports these things are far more important than in the real world of daily driving and the brakes only have to survive 500 miles or so on the racetrack...

In mountain climbing, on a Porsche, Ferrari or aircraft ounces count. On a 6000 pound and up truck that is going to tow more than double that in a trailer at a fraction of the velocity achieved on the track or in the air?

Not so much...sorry!

If the RWAL valve were stuck open the front brakes should still bleed and work so the pedal would not go all the way to the floor with no resistance-or at least it shouldn't. The proportioning valve should not be designed so that it allows a single point failure that would disable the entire hydraulic brake system (the whole reason for isolating the front and rear brakes from each other). That said, Dodge expected that truck to go to the crusher in 10 years so perhaps there are multiple failures in either the proportioning valve or the Master Cylinder...

If those new rear pucks are not dragging at all then the pistons have not been extended from the fully compressed position. If that is the case the cause is that insufficient pressure has been applied to the rear brakes to extend the piston which would indicate substantial trapped air remaining in the system.

I suspect that the culprit is air trapped in the back side of the hollowed out pistons. Since the rear calipers are probably not connected with a flex hose I suggest that you open the bleed valve and tap on the cylinder casting of the caliper with a rubber mallet to break the surface tension that is trapping the air and allow fluid to gravity feed into the air space remaining in the cylinders.

If that fails you may have to dismount the calipers, remove the bleed valves and dribble brake fluid into the cylinders a drop at a time with something like a syringe or needle point oiler through the brake line port while holding the caliper piston side down and the brake port slightly elevated.

Good luck!