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."

 

[Paisano]

Understood. Standard bleeders difficult to work with not having an assistant.
I'm considering options now. I need to get this solved by next weekend.
Thanks for all your input. It might not seem like it yet....but I can already see these responses are helping.

 

[Curious Hound]

Understood. Standard bleeders difficult to work with not having an assistant.
I'm considering options now. I need to get this solved by next weekend.
Thanks for all your input. It might not seem like it yet....but I can already see these responses are helping.

You can make a vacuum bleeder like @alwaysFlOoReD showed for next to nothing. Buy some plastic tubing from a hardware store and ask your grandpa for an old moonshine jar. Bonus if he leaves some moonshine in it for you.

Then, it's an incredibly easy 1 person job.

 

[Paisano]

I just looked at that engine vacuum bleeder again. That is some creative problem solving there.

 

[Uncle Gump]

Thanks for suggestion Gump. But I'm not a fan of gravity bleed.

It's ok... you gotta do you.

100% of the time I open up a brake system... it gets gravity bled. It probably works 95% of the time without ever pumping the brake pedal in a traditional brake bleeding procedure. It makes me a total fan and I do it without any assistance from another person.

A couple more things....

If you truly have the wrong length push rod. You would notice that with excessive free play at the top of the brake pedal whether the engine is running providing assist or sitting static without power assist. So I tend to think it's not your problem. Because you say the pedal feels good until you start the engine. Then you say the pedal goes to the floor which you stated... my power brakes aren't working. I say... it sounds like they're working. That's what power brakes do... they assist you in applying the brakes and you'll always see more pedal travel when the system is applying power assist. It's just how it works.

So why is your pedal so low?

In my opinion... based on my 25 plus years as a professional technician and another 15 years as a hobby... this happens from three things. Air in the system... a leak in the system or improper drum brake adjustment in the rear. Rear drum brake adjustment is critical to pedal height.

I know you say you adjusted them... but i feel it would be worth jacking up the rear and sliding under there with a brake spoon in hand. If you can get 6 or 8 clicks on each star wheel... you would be surprised how that will affect pedal height.

If it's quiet... truck not running... if you push the brake pedal and you can hear the rear shoes squeak or move on the backing plate... they're too loose.

I know you spent your hard earned money on those speed bleeders and you want to believe but... you already said one was leaking and needed replaced. I still say they should go in the trash.

Good luck.

 

[Paisano]

Thanks Gump.
I read your entire response and it makes sense. Good information...as always going back several years. And that's a major point you just made about the booster push rod adjustment, which I did not know. I would not be surprised if this was just stubborn air in the system. I'm going to order a couple tools off Amazon. I suppose one of those tools would do an 'assisted' gravity bleed anyway. Should get it some time this week.
I adjusted the rear drum brakes earlier this month, because of excessive brake pedal travel. That fixed it before all this happened