Spongy brake pedal tightens when braking in reverse before going bleeding back down

[Joey 7]

I have a 86 K5 Blazer. Recently swapped a 90s 14 bolt in, initially had almost zero brakes, changed the master to a 1 ton and changed the proportioning valve. I can at least stop now but nothing id drive in town with. So when I drive in reverse and slam on the brakes, my pedal hardens a ton. Then shortly after goes back to being spongy.

When I changed the axle I adjusted the shoes to barley touch the drum. My only assumption at this point is that the shoes are adjusted too far out, so when I brake in reverse they try to adjust before going back to normal. I'm really not sure, I wanted a second opinion before tearing everything apart. Thanks in advance

 

[fast 99]

Doesn't sound like adjustment. A few things come to mind, am sure others will have more.

Why change the prop valve? That is another part commonly replaced that usually doesn't fail. Is there some u tube video everyone is watching?

oversize drums, wrong size wheel cylinders, obviously air, over stroked master during bench bleed, also some master cylinders can trap air because of mounting angle, are wheel cylinders new?

 

[Joey 7]

Doesn't sound like adjustment. A few things come to mind, am sure others will have more.

Why change the prop valve? That is another part commonly replaced that usually doesn't fail. Is there some u tube video everyone is watching?

oversize drums, wrong size wheel cylinders, obviously air, over stroked master during bench bleed, also some master cylinders can trap air because of mounting angle, are wheel cylinders new?

I changed the prop valve because some times the rear would lock up and sometimes it wouldn't. Didn't make a difference obviously, it just seemed like something to eliminate.

The rear axle is out of a 97 2500 I Believe. Fully rebuilt and replaced everything. I did find one of the adjustments was slipping but I fixed it all and still no difference.

I am able to pump it and it firms up after 3 or 4 pumps. Once it hardens I can hold the pedal down and it doesn't ever bleed down, which leads me to believe it isn't air. I've bled the thing probably a dozen times by now and haven't gotten air in quite a while, gone through over half a gallon of fluid. Also, I'm typically able to lock up the rear way too easily. Even when the pedals super soft.

I tried adjusting the rears way tight and way loose and almost everything in between.

My pump sounds horrible too once I get the pedal stiff, so that's on the list to replace. I'm not sure if hydro boost could have anything to do with it.

I don't think the size of drums or shoes would have anything to do with it since I have the matching master. I possibly could've over stroked it? I know it was bled all the way through at least

Thanks for your input,

Doesn't sound like adjustment. A few things come to mind, am sure others will have more.

Why change the prop valve? That is another part commonly replaced that usually doesn't fail. Is there some u tube video everyone is watching?

oversize drums, wrong size wheel cylinders, obviously air, over stroked master during bench bleed, also some master cylinders can trap air because of mounting angle, are wheel cylinders new?

 

[fast 99]

3-4 pumps sounds like a seal leaking somewhere or air. Just because the pedal doesn't sink doesn't mean there isn't air. A sinking pedal would mean a leak either internally or externally. If the rear brakes are locking up prematurely, I would suspect an issue with the front 1/2. Has rear cylinder diameter been changed it may be contributing to the issue.

There is A LOT of engineering that goes into brake systems. Swapping mismatched parts around is asking for problems.

 

[Ricko1966]

How and when did this problem occur?

 

[Joey 7]

3-4 pumps sounds like a seal leaking somewhere or air. Just because the pedal doesn't sink doesn't mean there isn't air. A sinking pedal would mean a leak either internally or externally. If the rear brakes are locking up prematurely, I would suspect an issue with the front 1/2. Has rear cylinder diameter been changed it may be contributing to the issue.

There is A LOT of engineering that goes into brake systems. Swapping mismatched parts around is asking for problems.

Nope the cylinders are stock size as for everything on the rear axle. Right now everything I have in the truck is equivalent to a k20.

I've been leaning towards the fronts being the issue except everything worked fine before the new rear. Only thing I can think like you said is the shoes aren't engaging when they should

 

[Joey 7]

How and when did this problem occur?

Right after swapping in a 14 bolt ff rear in place of the stock 10 bolt

 

[fast 99]

With a proper pinch off tool clamp the rear brake hose. Is the pedal normal?

Do not use a vise grip or similar to tool.

You must be registered for see images attach

 

[Ricko1966]

Right after swapping in a 14 bolt ff rear in place of the stock 10 bolt

Everything worked fine prior? Did you replace rear shoes? 3 things coming to mind right now,except you've thrown a bunch of new pieces into the equation since. The 3 that come to mind are it still has air in the lines you've put your shoes on backwards BOB is the norm,but double check BOB means big on back meaning long shoe rear position,my other thought would be rear adjusters and initial adjustment. I hope this helps

 

[fast 99]

After doing what Ricko1966 suggests,

If the drums are oversize pedal will suffer, although usually not to the extent described. Reason for this is shoes have a different circumference than drum. Shoes have to conform to the larger diameter or bend. At this point you should start isolating parts of the system in order to narrow down the offending problem. Am also thinking if the rear cylinder is larger than the old one [likely is] it might be contributing to the issue.

Can you provide a picture of rear brakes?

 

[Joey 7]

After doing what Ricko1966 suggests,

If the drums are oversize pedal will suffer, although usually not to the extent described. Reason for this is shoes have a different circumference than drum. Shoes have to conform to the larger diameter or bend. At this point you should start isolating parts of the system in order to narrow down the offending problem. Am also thinking if the rear cylinder is larger than the old one [likely is] it might be contributing to the issue.

Can you provide a picture of rear brakes?

I never had any problems with the brakes in the couple years I've had the truck until now. Initially when this whole thing started I made some plugs, disconnected the lines to the cylinders and plugged them. When I did that I could not move the pedal more than 1/3 it completely stopped. I assumed it was just due to the fact the fluid has no where to go. I never tested the front.

Yes the cylinders are bigger, but so is the master. I've found plenty of forums of people that did this exact conversion without any issues with or without the k30 mc. That's kinda why I'm thinking I screwed up somewhere with the rear brakes. My only real drum experiences before this have been on air brakes. I'm pretty sure I put shoes on the right side but I'd have to double check, could worn drums cause anything like this? I know mine are pretty worn, I don't know how to measure them

Would a slipping power steering pump or hydro boost cause anything like this? I've always heard if they went out you'll just have a hard pedal like with a vacuum booster, but a guy I knew swore he saw this problem in the military and it was the hydro boost.

I'm out of town for work for the next week but I'll attach whatever photos I have. These are of the passenger side, which is the one the adjuster was slipping.

Again, thanks for your help.

 

[Joey 7]

I never had any problems with the brakes in the couple years I've had the truck until now. Initially when this whole thing started I made some plugs, disconnected the lines to the cylinders and plugged them. When I did that I could not move the pedal more than 1/3 it completely stopped. I assumed it was just due to the fact the fluid has no where to go. I never tested the front.

Yes the cylinders are bigger, but so is the master. I've found plenty of forums of people that did this exact conversion without any issues with or without the k30 mc. That's kinda why I'm thinking I screwed up somewhere with the rear brakes. My only real drum experiences before this have been on air brakes. I'm pretty sure I put shoes on the right side but I'd have to double check, could worn drums cause anything like this? I know mine are pretty worn, I don't know how to measure them

Would a slipping power steering pump or hydro boost cause anything like this? I've always heard if they went out you'll just have a hard pedal like with a vacuum booster, but a guy I knew swore he saw this problem in the military and it was the hydro boost.

I'm out of town for work for the next week but I'll attach whatever photos I have. These are of the passenger side, which is the one the adjuster was slipping.

Again, thanks for your help.

 

[fast 99]

The master cylinder has not been removed since this started?

Would think a slipping belt would cause a hard pedal. Haven't seen a hydro boost do that but never want to say never when it pertains to vehicles.

 

[Joey 7]

The master cylinder has not been removed since this started?

Would think a slipping belt would cause a hard pedal. Haven't seen a hydro boost do that but never want to say never when it pertains to vehicles.

I know my pumps slipping, so I have a new pump and a rebuild kit for the hydro boost that should be at the house by now. Don't really think it's causing it but I don't know that it isn't either.

I originally had practically zero pedal after I got all the air out the first time. Did some digging and found the master likely didn't have enough rod travel (if that's the right term). After changing it out for the k30 master with maybe 1/4 inch more travel I have okay at best brakes. Like I said I can pump it a couple times and it completely stiffens up.

The weird thing though is right where it stiffens up, before pumping I press the pedal down and at the exact same spot it would stiffen is where I hear the pump start slipping.

And The pedal usually feels about normal without the engine running, once i turn it over three pedal is super soft. Which makes me believe the hydro boost is working, and the pump just starts slipping under a certain amount of stress.

 

[fast 99]

There is only one or 2 reserve pumps in the hydro boost after that, the pedal will go hard. Be careful rebuilding the HB they can be dangerous if not done properly. That is one part I sent to a local rebuilder or bought a new replacement.