Archived Serious Braking Problem - HELP

[JMead]

I am experiencing a serious braking problem. It usually happens after travelling at highway speeds. I have looked for answers in the forums but haven't come across anything matching the problem I'm having.



Here's what happens: After getting on the highway and travelling varying distances, when I first hit the brake pedal, it is very stiff. Most times when I come off the pedal and then back on, the brakes function normally. BUT... sometimes I literally have to put both feet on the pedal and lift my 240lb a** out of the seat to slow the truck!!! NOT GOOD!!!



Then at other times, if I keep steady pressure on the pedal, the brakes start off stiff and then I feel the pedal "release" and function normally again.



Somebody - Anybody, Please Help!

 

[Missouri Mule]

From 96 service manual,

"Hard pedal of high pedal effort, A hard pedal or high pedal effort may be due to lining that is water soaked, contaminated,glazed or badly worn. The Power booster or check valve could also be faulty. "



The check valve mite only apply to vacuum operated boosters.

 

[JMead]

I just had the rear drums turned and new shoes put on. The check valve was replaced as part of a recall so unless the valve went bad it sounds like it might be the booster. The only thing that is strange is that the problem is intermittant. If the booster was bad shouldn't this be happening all the time?



Also, what does the "RAWL sending unit" do?

 

[Missouri Mule]

The CAB(controller antilock brake) "Gathers information from various inputs to control the rear brake system during an ABS stop. It monitors the ABS system for proper operation. " Located center of instrument panel.



Hydraulic control unit,"Provides modulation of the hydraulic portion of the rear brakes during an ABS stop. " Located under master cylinder and is mounted on a bracket with the combination valve.



Recall was for putting check valve inline with vacuum pump there is another one on/in brake booster.