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Brake help needed.

Gonna drag this back up as I still have the problem. To recap, I have a 1ton 14bff and a 3/4ton 10 bolt swapped under my 88 jimmy(note i used the new calipers off my 6 lug 10 on the 8 lug 10bolt). I currently have a VERY spongy peddle. I have changed the calipers, rubber lines in the front, one leaky wheel cylinder, adjusted the back drums, swapped the MC and prop valve with known good working ones, and bled about 4 or 5 quarts through the whole system today. I still have a very spongy pedal and when I step on the brakes more than a normal stop I get the back brakes grabbing. What Concerns me most is the lack of pedal even after swapping all those parts and bleeding the system.

I'm wondering if anybody has any more direction for me to take? This is getting frustrating and if i cannot fix it before we get snow that stays, I'm gonna have to find another mode of transportation.
 
Have you tested the booster?

Test 1

With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the booster is working properly.

Test 2

Run the engine a couple of minutes.
Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster.

Test 3

Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight.

Inspect the Check Valve

Disconnect the vacuum hose where it connects to the intake manifold. Do not disconnect the vacuum line from the booster. Air should not flow when pressure is applied, but should flow when suction is applied. If air flows in both directions or there is no air flow, the valve needs to be replaced.
 
have the front rotors been turned? if not, they may glazed over and not grabbing the pads right. Might need to upgrade the mastercylinder. does it dribble out or blast out of the front bleeders? do the calipers slide in and out on the pins fairly easily? you might need to put some antiseeze on the caliper pins or get new pins.
 
Booster is holds vacuum after sitting for a long time so i think its alright but I will try your test.

The rotors have not been turned to my knowledge and this problem was there before with the original diff. Fluid shoots out the bleeders and the calipers are new and working correctly.


I don't think these think either of those would cause a lack of pedal. But I don't know.
 
Ok In my on going quest to make these brakes work I got some new info. I bled the brakes again, that didn't help at all. I did note that the front resevoir on the MC was the one going low, is this correcct for a 1/2 ton. Now the fun part is, if i pinch off the the rubber line going to the rear brakes, the peddle firms right up. Now I'm thinking that the wheel cylinders on the 14 bolt(with the LARGE drums I think 13"?) are requiring too much fuild before engaging the drums. Would a 1 ton MC or an adjustable prop valve help out this situation at all or is my thinking all wrong? Any advise is very welcome.

Thanks
 
If the MC volume was too small you would be able to pump the brake pedal and come up with good brakes. If this happens then a 1 ton MC might help.

In case you don't know, when you bleed the brakes you shouldn't reuse the brake fluid that you bleed out. Brake fluid holds air so if you put the old fluid back in it defeats the purpose.

Good Luck,
Mike
 
A JB6 master is a light-duty 3/4-ton, which does not use the 13" drums.

JB7 or JB8 used the 13" drums... these trucks also (usually) had the load-sensing brake valve & linkage over the rear axle.
 
wow, you have been having this problem for awhile now,,,,,

you are having the same issues i was having when i did the 14bff swap with the rear locking up, i was always under the impression, and it's been mentioned on here before, that the rear brake shoes are MUCH larger in surface area than the stock 10bolt shoes were, basically giving you much more stopping resistance out back...

I personally did the disk swap also, still have a semi-soft pedal, but once those pads are heated up (to a certain point) they work REALLY nice :D

nope, never did figure out why mine grabbed so hard either, i just figured it needed either the 3/4-1 ton MC or a prop valve from the same setup to move the proper amount of fluid needed for proper braking...

just my thoughts though :p:
 
Juat a thought, you may have done it, but I didn't see it mentioned. Did you bench bleed the master cylinder before installing it and make sure it didn't go empty after that?

Other than that, I've gotta agree with some of the advise above. Your mixing 3/4 ton brakes with a 1/2 ton master and prop. valve. You might want to try locating the prop. valve and master that went with those brakes.

I did a 1 ton brake swap on my Burb. and went with the 1 ton master and prop. It stops like my 1 ton trucks.

Good Luck,

Eric M.
 
norville said:
Are your rear brakes adjusted all the way up. Crank them till it stops the drums. Then bleed.

bob
Click to expand...

That's a good way to overheat your brakes and warp your drums.

Drum brakes should be adjusted out until they just touch the drum. Or, in a perfect world, adjusted out until the drum stops (to center the shoes) and then backed off until it rotates freely. They should definitely be adjusted, though. In a disc/drum system, the drum brakes are what controls the pedal height.

I concur with the prop valve diagnosis, especially since it was doing it even before the swap. A disc/drum prop valve (actually called a combination valve) does two things:
1. It retards the flow of fluid to the disc brakes. Drum brakes have a greater distance to travel before they contact the braking surface than discs, so the valve sends fluid there first. This the metering function of the valve.
2. It limits the pressure going to the rear brakes to prevent rear wheel lockup. This is the proportioning function of the valve and appears to be the proplem with yours.

It also serves a third purpose of housing the pressure differential warning switch but that's more of a secondary function.

Before you go buying new parts, try taking the combo valve apart and cleaning it out real good. It could be something simple. If all else fails, then try a new valve. You could try a stock replacement but, as previously mentioned, aftermarket adjustables are cheaper and can be beneficial for when you carry a heavy load or pull a trailer. However, Note that adjustable proportioning valves are not DOT approved for street driving!
 
For those who have the rear discs and are using a drum disc master then the rear discs back off too far and can make for poor braking due to the pressure differences required in disc vs. drum.

I put one ton master and booster on mine and couldn't get good pedal.

the key for me was a good booster my new one was bad from the parts store and the big improvement came when I did the adjustable proportioning valve and eliminated the stock one. I also put in a residual pressure valve in the rear line near the master. My pedal is good and solid and I can skid straight to a stop from 50 mph. Not much fluid goes to the rear vs the front to avoid it locking up.

Also a big problem for me was bleeding. the calipers were put on top of the rear rotors to keep them out of harms way and no matter how I did it air would get back in so a friend at his shop took the calipers off and bled them with the bleeders down using a block of hard wood that was the same thickness as the rotors.

I got my valves from Jeg's if I remember correctly.
 
Well I wanted to update this thread as I've been working on my truck a bit lately.


I've finaly got around to working on my truck after buying a house and some other things. What I've done is put a 1 ton master cylinder on the truck leaving the factory half ton booster. I have also put an adjustable prop valve in the rear after the factory valve. Now what i've got is very little pedal but when the adj. prop valve is set to the least amout of rear breaking the breaks work in a predictable manner. I'm still having trouble getting pedal though. I compared it to my buddies 91 halfton and it feels like I should have a bit more pedal. I've bled the breaks many times even doing a gravity blead all the way around as well as bench bleading the MC. Any Idea's on what could be providing the poor pedal now. As well I have the big steel line hooked up to the front port (smaller resevoir) and the small steel line hooked up the the rear port (larger resevoir). This is how they were hooked up when I got the MC and the way they were hooked up on my old mc. Can some one verify that this is the correct way to have them hooked up on the 1 Ton MC?
 
goldwing2000 said:
That's a good way to overheat your brakes and warp your drums.
Click to expand...



perhaps I didn't make myself clear. Goldwing you are correct. I was refering to the time during the bleeding process. To crank up the shoes so they cannot actuate while pumping. At least that's what worked for me. After it was bleed I adjusted the rear brakes correctly. I was un able to get them - non-spungy till I did this.

bob
 
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