Chevy brake issue

[DaveB]

I am trying to fix a brake problem on the old suburban project. This is one of the reasons I got the old project back because they spent a bunch of money on trying to fix the issue. I haven't started to dig in too far yet but the brake pedal is very soft for the first half of the travel then goes very firm to the point of "clunking". They had a shop replace the master cylinder, front calipers and supposedly bled it many times. I checked the brake booster push rod and travel to the master cylinder and that looked correct. To me it acts like the front brake circuit is messed up somehow. Any ideas before I dig into the issue? Proportioning valve maybe ?

 

[Hickey]

I was gonna say proportioning valve makes sense.

 

[Jinx]

I know a lot of people have pretty good luck with just pulling a master cylinder out of the box, installing it and traditionally bleeding the brakes.

I have NEVER had that luck. If I don't bench bleed it, it always haunts me with things like you are talking about until I bench bled the master cylinder then reinstall it...

I think it has to do with getting all of the random pockets of air out of the master cylinder before you flush the air out of the lines...

 

[DaveB]

I know a lot of people have pretty good luck with just pulling a master cylinder out of the box, installing it and traditionally bleeding the brakes.

I have NEVER had that luck. If I don't bench bleed it, it always haunts me with things like you are talking about until I bench bled the master cylinder then reinstall it...

I think it has to do with getting all of the random pockets of air out of the master cylinder before you flush the air out of the lines...

I was just talking to my son about this as a possible issue.

 

[NYCEGUY01]

Does it act the same with the truck running vs. not running.
Soft then harder.

 

[DaveB]

Does it act the same with the truck running vs. not running.
Soft then harder.

Yes

 

[NYCEGUY01]

If you pump the pedal does it change . If you pump it a few times does the initial softness go away ?

It’s been a few years but I fought a similar issue on an old Chevy c30. Chevy had several different brake options these years although the masters and boosters and calipers all basically look the same and will bolt right on, the wrong combination of parts will cause some issues. Mixing the wrong front calipers with the wrong master cylinder will have you scratching your head for sure. If the glovebox sticker is still there it will have the correct code ( jb4, jb5, jb6 etc..) and you want to make sure you actually have the correct master and calipers that match that. I think if you look up the parts on Oreillys it will still show that, rock auto prob will as will. Some places just the parts by year and model and it’s easy to get the wrong stuff.

I used to have all this in my head but it’s been a while since I did a lot on these trucks. I just started in on another one so some of its coming back..lol
I remember there are some calipers with larger or smaller pistons and definitely masters with different bore sizes but externally they are pretty identical.

If you were just driving it one day and this started it could be ruled out but with someone else in possession of the truck and a shop swapping parts around already it’s hard to say. It may have had a bad master and the shop installed the wrong one just making a new problem to replace the old one.

 

[DaveB]

Pumping it doesn't change the problem. The question on what parts they used is a good possibility. This old Suburban was one of my franken truck projects. The frame and running gear came from a 78 and the body is from an 85. I think I used the brake booster from the 78 to match the running gear but its been over 20 years since I built it. If the shop looked at the glovebox sticker they would have used 85 parts not 78 parts. I looked on Rock Auto and the 85 and 78 use different master cylinders. I will have to do some part number checking.

 

[NYCEGUY01]

Pumping it doesn't change the problem. The question on what parts they used is a good possibility. This old Suburban was one of my franken truck projects. The frame and running gear came from a 78 and the body is from an 85. I think I used the brake booster from the 78 to match the running gear but its been over 20 years since I built it. If the shop looked at the glovebox sticker they would have used 85 parts not 78 parts. I looked on Rock Auto and the 85 and 78 use different master cylinders. I will have to do some part number checking.

I think you are on to something now sir...
Look up those 2 part #'s and I bet they have a different bore size etc.... I use oreillys site alot for this stuff as they seem to list all that info nobody else does and its quick and easy. Especially if you already have the 2 part #'s.

On the old Chevys you can usually get away with blindly swapping most parts.
Every once in a while you get "lucky" and accidentally find the right combination of parts that causes a problem though...

 

[85CUCVKRAWLER]

There is a proportioning valve right in the middle of the foremost cross member. Right underneath the radiator.

This proportioning valve is really annoying to deal with. It has a safety feature internal to the unit that locks up the rear brakes if it "determines" that a brake line has been cut. It is spring operated utilizing a weird banjo fitting. There are instructions online on how to reset this valve to continue bleeding the system. I forget how to do off the top of my head, i had to do it one time when on my K5 when i was experiencing issues like yourself, except i replumbed my rear brakes to match the 14 bolt install.

So i would look up how to reset this valve.
Reset the valve per isntructions.
Bleed the rear brakes at the cyliners.
Bleed the front brakes at the calibers.
Then see where youre at.

Be forewarned, if you dont have the hydroassist brake system, your brakes are always going to feel kindof spongy compared to modern systems. Unfortunately its just the way these ancient systems operate off of just engine vacuum. If you want that more "modern" feel the brake system you will have to scavenge a hydroassist setup from the junkyard (its why they changed designs in the late 80's).

 

[DaveB]

I think you are on to something now sir...
Look up those 2 part #'s and I bet they have a different bore size etc.... I use oreillys site alot for this stuff as they seem to list all that info nobody else does and its quick and easy. Especially if you already have the 2 part #'s.

On the old Chevys you can usually get away with blindly swapping most parts.
Every once in a while you get "lucky" and accidentally find the right combination of parts that causes a problem though...

I looked up some of the master cylinders and calipers on Rock Auto for both years and the bore size for the master cylinder is different and the calipers come in different piston diameters. To further complicate this the 85 body was off of a 1/2 ton and the 78 was a 3/4 ton. That makes a difference on the diameters and bore sizes. I'm going to have to pull some parts and see what they ended up putting on. I'm not sure the shop knew enough to check for all of the options.

 

[DaveB]

There is a proportioning valve right in the middle of the foremost cross member. Right underneath the radiator.

This proportioning valve is really annoying to deal with. It has a safety feature internal to the unit that locks up the rear brakes if it "determines" that a brake line has been cut. It is spring operated utilizing a weird banjo fitting. There are instructions online on how to reset this valve to continue bleeding the system. I forget how to do off the top of my head, i had to do it one time when on my K5 when i was experiencing issues like yourself, except i replumbed my rear brakes to match the 14 bolt install.

So i would look up how to reset this valve.
Reset the valve per isntructions.
Bleed the rear brakes at the cyliners.
Bleed the front brakes at the calibers.
Then see where youre at.

Be forewarned, if you dont have the hydroassist brake system, your brakes are always going to feel kindof spongy compared to modern systems. Unfortunately its just the way these ancient systems operate off of just engine vacuum. If you want that more "modern" feel the brake system you will have to scavenge a hydroassist setup from the junkyard (its why they changed designs in the late 80's).

I will have to look up the reset procedure. On some of the ones I have fixed I just had to release the line pressure on the circuit that wasn't opened up.

 

[NYCEGUY01]

There is a proportioning valve right in the middle of the foremost cross member. Right underneath the radiator.

This proportioning valve is really annoying to deal with. It has a safety feature internal to the unit that locks up the rear brakes if it "determines" that a brake line has been cut. It is spring operated utilizing a weird banjo fitting. There are instructions online on how to reset this valve to continue bleeding the system. I forget how to do off the top of my head, i had to do it one time when on my K5 when i was experiencing issues like yourself, except i replumbed my rear brakes to match the 14 bolt install.

So i would look up how to reset this valve.
Reset the valve per isntructions.
Bleed the rear brakes at the cyliners.
Bleed the front brakes at the calibers.
Then see where youre at.

Be forewarned, if you dont have the hydroassist brake system, your brakes are always going to feel kindof spongy compared to modern systems. Unfortunately its just the way these ancient systems operate off of just engine vacuum. If you want that more "modern" feel the brake system you will have to scavenge a hydroassist setup from the junkyard (its why they changed designs in the late 80's).

Ive never had any issues with the factory ones but I had an aftermarket one give us fits once. It may just be Ive been lucky though. I always thought you just loosened up the lines and pressure and they would reset and the little valve would return to the middle on its own.
His may have tripped and be bad though. It is over 40 years old...lol

 

[Hickey]

I looked up some of the master cylinders and calipers on Rock Auto for both years and the bore size for the master cylinder is different and the calipers come in different piston diameters. To further complicate this the 85 body was off of a 1/2 ton and the 78 was a 3/4 ton. That makes a difference on the diameters and bore sizes. I'm going to have to pull some parts and see what they ended up putting on. I'm not sure the shop knew enough to check for all of the options.

My first Blazer was like that. It was titled as a 76, but had all kinds of surprise parts from an 84.

 

[85CUCVKRAWLER]

Ive never had any issues with the factory ones but I had an aftermarket one give us fits once. It may just be Ive been lucky though. I always thought you just loosened up the lines and pressure and they would reset and the little valve would return to the middle on its own.
His may have tripped and be bad though. It is over 40 years old...lol

There is a plunger you have to pull out, but not all the way, just a little bit (banjo). Then i think you have to pump the brakes. Then the safety piston will reset itself. Then you can release the plunger. Like i said, its a safety feature incase you lose a brake line, so the front brakes will still work.

 

[85CUCVKRAWLER]

I looked up some of the master cylinders and calipers on Rock Auto for both years and the bore size for the master cylinder is different and the calipers come in different piston diameters. To further complicate this the 85 body was off of a 1/2 ton and the 78 was a 3/4 ton. That makes a difference on the diameters and bore sizes. I'm going to have to pull some parts and see what they ended up putting on. I'm not sure the shop knew enough to check for all of the options.

I would be worried about matching the booster with the master. 1/2 ton to 1/2 ton, 3/4 ton to 3/4 ton. The pedals dont matter neither do whats at the axles. Unless youre going full one ton, even then i wouldnt be too worried. I have a stock k5 master and booster on one tons and it stops just fine.

 

[GER4x4]

There's a fellow who goes by the name GubNi on Pirate 4x4 who knows everything there is to know about brakes, and is a brake parts supplier. I've purchased parts from him in the past and he has always been helpful ad cooperative. If you haven't already found and answer I would suggest contacting him.

 

[DaveB]

Thanks for the tips everyone, now I just need to dig in and see what I find.

 

[DaveB]

Did some checking and found that the proportioning valve is needing a reset. There wasn't a dash light on but the emergency brake didn't turn the light on either. Busted out the ohm meter and the valve switch connector is grounded. The dash light must be burned out. I also decoded the VIN numbers and found out that the 78 was actually a half ton with 3/4 ton axles put under it. I will need to get the numbers off the axles to figure out what they came from. It also looked to me like the front calipers were not new but the pads were. Not sure how many of the supposedly new parts are actually new.

 

[DaveB]

I got my son to help me do a good old fashioned brake bleed and found a bunch of air in the right rear wheel cylinder. After bleeding the two rear lines I started on the right front. Bleeding that one reset the proportioning valve. The last remaining issue is the bleeder on the left front caliper. It doesn't seem to want to let fluid out. The brakes are much better now but I still need a new bleeder for the left front.