trying to solve solve soft brake pedal since weekend

[pentode]

Did the pedal go to the floor before you swapped out the master cylinder? I wasn't too clear on that. I've had bad master cylinders cause similar symptoms, including one new out of the box.

 

[Paisano]

I just tried it. The power brakes won't stop the truck. I have to stop the truck with the parking brake.
Maybe it will stop the vehicle if I push the pedal ALL the way down. And I won't do that because it will damage the master cylinder like it has in the past.

Yes the pedal was slack with the old master cylinder too. And I already adjusted the drum brake star wheels in the last 2 weeks.

 

[Paisano]

The brake pedal feels EXACTLY the same before and after master cylinder replacement.
It's just loose.

 

[RobbieD]

If the parking brake will stop the truck, it sounds like the rears are adjusted, at least somewhat.

The big question in my mind is, engine off, or no vacuum, you have a hard pedal; but does pressing the pedal actually engage the brakes?


edit: grammar

 

[Paisano]

The rear brakes are adjusted fine.
Engine turned off.......truck rolled slowly in neutral......brakes will stop truck

Air in brake lines will cause spongy pedal in power brakes and non-power manual brakes........correct?

brake booster push rod ?

 

[RobbieD]

I honestly don't know. This is a strange one.

It's basically a mechanical link (rods) from the brake pedal to the master cylinder; vacuum should only push it harder. There is a vacuum bleed valve, internal in the booster, which may have an effect on this.

But, I am also curious about whether your booster push rod is close, or way out of spec.

Gimme a few; I'll post a description of how the booster works, from the manual. The more info you have, the better equipped you'll be to find the problem.

 

[Uncle Gump]

Throw the speed bleeders in the trash... all of them.

Put some standard bleeder screws in it... then gravity bleed it. Or bleed it anyway you feel comfortable.

 

[RobbieD]

I just tried it. The power brakes won't stop the truck. I have to stop the truck with the parking brake.
Maybe it will stop the vehicle if I push the pedal ALL the way down. And I won't do that because it will damage the master cylinder like it has in the past.

Engine turned off.......truck rolled slowly in neutral......brakes will stop truck

Three thoughts:
1. Bleed the crap out of the brakes. Until that's done, we can't rule out air in the system. Get this ruled out.
2. Check the booster push rod adjustment. Also get this ruled out.
3. That leaves, something is screwed up in the booster.

A couple of things stick out here. First, the factory says only two things on the booster can field-serviced- replacing the check valve and adjusting the push rod. Second, the manual talks about the "booster rubber reaction disc" like it's really important (I red-boxed that part). If the booster rod was pulled out in the past, or that reaction disc got screwed up, it sounds like the brake application pressure can also get screwed up.

Posted below are three pages from the 1992 manual, that describe how the booster works. I've done a lot of brakes over the years on these trucks, like the others posting here have, and I've never run into this problem. We can only go so far in troubleshooting over the internet. You have the truck in front of you, and know what's been done so far. You will figure it out, and you will get it working right.

And when you do, I'd love to know what it was.

 

[Paisano]

I read all these pages you posted.
Thanks for suggestion Gump. But I'm not a fan of gravity bleed.

I'll probably go this route
inexpensive booster pushrod adjustment tool off Amazon

https://www.amazon.com/s?k=brake+booster+adjustment+tool&channelId=500&clpRedir=Y&plpRedirect=mhFallback&tag=959media-20&tag=959media-20

pressure bleeder

I probably won't solve this problem this weekend.

 

[Curious Hound]

I read all these pages you posted.
Thanks for suggestion Gump. But I'm not a fan of gravity bleed.

I believe the PRIMARY point he's truying to make is "replace all of the speed bleeders with normal bleeders."