Helping my friend with his truck. Complained of soft sinking pedal, presumed easy fix, couldn’t be more wrong

1996 ram 3500, 2wd, rwal

Replaced both calipers (yes they are oriented correctly) new front hoses, new master (2) and booster, new wheel cylinders, adjust rear brakes. I tried bypassing the rwal unit but no difference so I’ll put it back in line. Suspect combination valve but don’t know of a way to diagnose and don’t want to start throwing junkyard parts that could also be bad. May need another master cylinder the sinking still occurs with a somewhat firm pedal. I’ve spent hours bleeding this truck so I should have a good pedal but it just doesn’t. Feels pretty good until engine is started then just falls to the floor. I also noticed a very weak flow of fluid through the rear bleed screws and when opening to gravity bleed I get almost no flow at all. Any advice on procedures or testing is greatly appreciated and a list of things to check is helpful as well. I’ll answer any questions you have to the best of my ability thanks guys in advance.

 

Still have a leak somewhere, air in the lines, are bad m/c.... I know you have replaced it a couple times, but, was it the same brand replacement? Separate the m/c from the booster, (don't disconnect any lines) just enough to see if you are getting fluid out the end of the master.

 

Did you bench bleed the new MC? I didn't when I was putting on a new one on a friends truck. Was talking too much and overlooked this.lol. Sounds like front is getting fluid but rear isn't. It did the same thing to me. After bench bleed. Both had good fluid movement.

 

I did bench bleed. I’m really leaning towards a bad master cylinder. Both that were installed were duralasts from AutoZone. Do any of you guys have a recommendation of master to buy. I was thinking of picking up a Napa one.

 

Since NAPA was bought out a corporation it seems their product has taken a down turn. Last calipers I bought looked like something the they were really cheap. The bleeder screws were different. One was real small and wouldn't even break loose and the other one was so tall that I couldn't get access to because it hit the brake line. I had to get new ones so I could use them. So sad.

 

It's been 20 years but I got a run of master cylinders from AutoZone that would all pool a not-insignificant amount of fluid in the bore when bench bleeding.

I finally started bleeding them in the store on their little bench in back and all 5 in stock did this. The manager agreed they were bad and said he'd return them all.

I think these were all remans but it's been a long time. I believe I finally ordered a new one...maybe a Wagner?

I guess I could understand a tiny bit of moisture in the bore but if it develops into drops/drips I can't see how that won't be hard on a booster AND ya gotta wonder what those seals are doing when you mash the pedal in a panic stop.....?

 

RockAuto shows a new Raybestos at $85. That would probably be my choice.