1996 Dodge 3500 Trk - Diesel Dually No Brake Pedal

[LB]

1996 Dodge 3500 Diesel dually, 4x4 no brake pedal.

Customer brought this truck in, hub bearings, front pads, calipers and master cylinder have all been replaced, if you hit the pedal hard it is rock hard, if you apply gradual pressure to the pedal it goes straight to the floor. Customer had it to a diesel shop which told him to replace his front pads and calipers, however, this did not fix his problem.
Not exactly a vehicle I would want to be driving if it won't stop...

I would appreciate any ideas on this one.

Thanks in advance.

[Tim Martin]

What is the pedal like if the engine is not running and all vacuum is exhausted from the booster? These trucks have had numerous brake issues.

[LB]

The pedal is right up and hard with out the vehicle engine on. If the truck is running apply brake fast get hard pedal feels as tho no vacuum assist. truck running, light application to brake pedal, the pedal slowly fades to the floor. Master cylinder has been replaced twice with new units.(not rebuilt)

[steven kiser]

scan for a.b.s. codes. it sounds like it has a bad pressure valve. hard pressure on the pedal will force valve closed and keep pressure. low pressure will allow the valve to remain slightly open and allow the pedal fade. i'm pretty sure it's a non serviceable unit and has to be an r&r. if you do this you will need a pressure bleeder or a drb unit to activate the abs pump to properly bleed the system.

[mopwrforme]

if its a diesel it can be set up with a hydro boost system where the p/s pump makes the power assit for braking. for a hard pedal i would ck. p/s fluid level, also preasure for high side hose, flow, if thats the case. the other option is a vacume pump setup which mounts. before the p/s pump to the back of the timing cover there have been several cases where oil will get in the vac. system threw the pump and sweel all the connectors 1st pull off the main vac. line at the pump and verify there is good vac then pull off the port at the booster behind the ck. valve for good vac. if not retrace the line untill you find the problem. fyi do the dash vents work properly. they run off the vac. pump as well. also chrysler recomends bleeding the brakes normally then actuating the abs to purge all the air from the abs pump then after actuation rebleeding them again. good luck ill ck. back in tonight if you havent fixed it. keep us informed thanks