77 GMC brake issue

[cam_114]

Two questions I'm seeking answers for..
1: part code for brakes "jb3" or "jb5"
I've looked all over the truck and internet, but no answers. I have available pics if they help.
The parts I may need are master cylinder and vacumn booster.

2: Brake light started coming on about a month ago, it would come on while sitting at a redlight. To disengage the light I'd shift to neutral, push the brakes..about half way or three quarters down, a sort of clunk noise would happen and light would disengage...
Then one day it just stopped.
So on the 16th I had both front and right rubber hoses at a local shop. They didn't bleed the brakes good at all. So I did them again.
Now the brake light started coming on again, this time its very hard to stop the truck,a lot of pressure is required, and I'm thinking only the rear brakes are engaging because of the recent snowfall and sliding to the right and zero traction braking.
I've bled them again and still same issue, and zero air bubbles.
I've tryed to diagnois the issue, I've started the truck and pushed the brakes, there is a hissing noise coming from booster.
Another thing to note, it appears fluid has, or did leak where the master bolts onto the booster, though I've never seen the master lose any fluid. Once when I first bought it the rear was low so I topped, no low fluid since.
I don't have a vacumn tester available to test it. I've visually checked the line, it seems good.

So any more tests to narrow the problem would be great...
I'm thinking its likely the booster, and or mc or possibly faulty rubber brake hoses...
Pics can be available...

Truck is c15 - 350 V8 front disc brakes, rear drum brakes.
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[motornut]

Two things I'd check
Any sign of a leak back of master where it mounts to booster
Sometimes people only see paint peeling off the booster
But you can remove the bolts holding the master to the booster and look inside where the rod pushes
Make sure the rears are adjusted right

The master seals go/ leak inside and it's pushing lower then it should making the light go on
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[tard01]

I didn't see you reference checking the brake light switch. The lights coming on shouldn't have anything to do with your actual braking. Have you checked your 'resting' position is properly contacting the switch? Maybe you just need to adjust/replace the brake-return-spring.

the part should be in the movement path of the brake pedal. Look under the dashboard directly above the brakes line of travel for something that looks like this:
http://images1.carpartsdiscount.com/..._oem_sls66.jpg


you'll probably just see the white tip of the switch. when it is depressed, the lights should go out.

also, make sure you arent mistakingly saying "brake lights" for running lights... you should keep int he back of your head the possibility of something acting up like a backup/neutral switch, or a bad light switch. i bet you're just not contacting the brake switch properly, but keep these other 2 in mind.

[tard01]

Also, have you tried gravity draining your lines? might have some built up crud that needs to come out.

[cam_114]

I havn't gravity bleed the brakes. I can't see how that'd make a difference between pressure bleeding. Wouldn't pressure bleeding be better for cleaning out crud?

The Brake light I mention is the one located on the dash. So the switch will have nothing to do with it.

I'm going to install a new MC and Booster as soon as I can.

I've also found out what the JB3 and JB5 brake codes meant...
JB3 is for 1" rotors and have weight rating for 4600lbs
JB5 is for 1.25" rotors and have weight rating for 6000lbs

From what I gather, it's best to go by your front disc brakes for which brake part code you'd use.

I will update with pictures/info on if changing the MC and Booster solved the problem, and if in fact it was seal between the booster and MC.

Thanks for your tips and advice. Cheers.

[tard01]

whoa whoa wait....

the brake light is triggered by the proportioning valve..... not by the master cylinder/brake booster.

do you feel confident you're getting full pressure front and rear?

have you checked to see if you're getting a voltage high from teh proportioning valve connector?

the valve could be "stuck" in which there are a few tricks to clear it, the connection could be bad, or it might be tripping erroneously.

go to the prop valve, remove the connector, and take a probe to it and see if it reads high or dead.

[cam_114]

Before the brake light started coming on I was getting good pressure.

After it started coming on the pressure in the front was low to zero. I'm confident there's still pressure at the rear, it seems those are the only brakes gripping. I almost slammed into a truck the other day, so i changed lanes, blew the stop sign and made a right turn. Initialy the truck didn't sway left or right until I changed lanes, once in other lane the truck pulled to the right.

So either the pressure is extremely low all around, or low to none in the front and some pressure in the rear.

I will check the connection tomorrow morning first thing.
Should I be testing the connection right on the valve, or at the braket??

[tard01]

Under that connector is a metal pin.. probe that. If u get a voltage, combined with what u just described, ten your problem is the proportioning valve... Often times u can fix it with no money and minimal effort
Other test.. open a front blessed and rear bleeder... Have someone under truck and someone slamming on the brake... blessed = bleeder btw.. phone is acting up.

If when they slam on the brake, you should see even squirt from front and back... If you only see the back squirt and the front dribbled, then your proportioning valve is the problem most likely. Note that doing this test does introduce air into the line, but its a simple way to confirm your problem.


There are some simple ways to clear the proportioning valve I can post tomorrow when i have access to a computer again.
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[tard01]

This post was originally from Hatzie a few weeks ago. So give them the credit for it.


You used to be able to buy fittings and a coil of line.

New Combination/Proportioning valve. http://www.ebay.com/itm/73-77-Chevy-...-/190540606447

Test Combination/Proportioning Valve from a detailed writeup on troubleshooting brakes. http://www.classicperform.com/TechBo...t.htm#testprop

Quote:
Use a test light by attaching a clip to a positive contact on the vehicle and touch the point of the tester to the electrical connection of the combination valve. If the the light does not come on, the valve system is operating correctly and no further testing is required.
If the light does come on, this indicates that the pressure differential valve is stuck in the front or rear position.
Bleed the brake system to determine if the front or rear lines are blocked off. Set up one front wheel and one rear wheel for bleeding at the same time. Crack both bleeder screws and gently pump the pedal a few times.
The blocked side will trickle fluid out when the bleeder screw is cracked and the pedal pressed. An unblocked line will squirt fluid out the bleeder.
The lines that are clear must be left open and the blocked lines should have the bleeder screws tight to cause pressure to build up on that side. Be sure to use the standard bleeding procedures to prevent air from entering the system.
Slowly press the pedal with steady pressure a number of times until the light goes out; this will center the differential valve. You may also hear a pop come from the proportioning valve. This is the metering valve returning to its equalized position. When the light goes out, close the bleeder screw. (See fig. below)
__________________

[tard01]

i should also add - if you're leaking brake fluid from the MC onto the face of the booster, it's not a bad idea to go ahead and replace it (and keep in mind new MC's are often failed themselves)

i wasn't trying to steer you away from that part. just trying to keep you from buying a brake booster if it wasnt necessary.

getting 0 pressure back isnt a brake booster problem i'm familiar with.... usually they either have 0 vacuum and are hard as a rock to press, or they work and if they do glide effortlessly to the floor, then you'll slam on the brakes hard. Doesnt sound like either of those issues fit your problem. so i was trying to steer you away from doing that replacement since it costs quite a bit more money.


if you wanted to test the booster is actually moving, you can dismount the MC and just press the pedal and visually inspect if it is driving teh rod forward or not.


MC is a cheap replacement... fixing the proportioning valve without spending a dime is the cheapest ;p

[cam_114]

Do I test the valve with truck running or not?

[tard01]

honestly, i am not 100% with the brake booster on there. i don't have a booster, so not sure how the lack of vacuum would influence that test.

i would think doing it with engine off would be completely sufficient, but i'll tip-toe around that one.




secondary question for you...
when you're bleeding the brakes, are you using 1 person or 2?
how is the pedal presser doing their job?

i had optimal results after battling these trucks brake lines by having them absolutely full strength press (controlled/slow) down to the floor and stick it as deep as their leg can make it move and hold it for a solid 10+ seconds before sealing/releasing and going again. this took me 6 pumps on a very airy line to clear, typically 2-3 normal. but that extreme extreme pressure and holding it for 10+ seconds allows those micro bubbles to keep sliding down their tubes. just make sure you don't run out of fluid while doing this.


also... i assume your truck is the same as mine....
the correct order to bleed them is illogical...
for the rear, you do the DRIVER FIRST, then the passenger... the reason is because the flex line drops in on the passenger side of hte axle. while the difference is neglible, if you're seeking the right way, that's the right way.

then for front, it's the standard.... passsenger side first, then driver side.


i recommend doing the rears first because that is what gives you that more natural brake pedal feel and makes tuning the fronts a bit easier i think.

also, have you properly adjusted your rear adjusters? are your wheel cylinders in good shape? not leaking?

my brakes were screwy because of the wheel cylinders leaking fluid and hence letting air in.

[cam_114]

2 person, and they held the pedal own as hard as possible.

the rear calipes were leaking awhile back, but i havn' had an issue with them lately. The fluid has stayed at the fill level every time I check...The cylinders arn't leaking to my knowledge.

My tester sems to be not working, I tryed it directly on the battery and no light. I think i'll go ahead and do the bleeder test and see where that takes me.

[cam_114]

Havnt adjusted the rear adjusters at all.

[tard01]

having the rear adjuster turned properly can make a world of difference in how they feel. make sure when you're testing though that you're pressing the drum full on while spinning or you'll get misleading results.

you can always use a light builb to test with instead of a tester... hte tiny little nubs that stick out on the side are the negative terminals. the bottom being the positive.

[cam_114]

So I tryed the steps to reset the prop valve. My passenger rear bleeder is outputting zero fluid. Bummer...
The driver rear seems to be outputting very little fluid also.

The front two were pumping it out nicely.

Wishing I had a heated garage for this work, not fun in cold weather.

[tard01]

just to check.... does your master cylinder have the rear or front closest to the firewall?
(should be the bigger tube)


well... on the bright side, it seems you may have found your problem is not the brake booster (costly) or the master cylinder (i'd assume at this point).


if you're planning on replacing the master cylinder regardless of this problem due to it leaking... i'd recommend replacing it prior to even buying a proportioning valve... that way if it miraculously fixes the problem, you saved 90 bucks on a proportioning valve... plus, those brass fittings on the brake lines can strip easily on a rusted proportioning valve, and it could open a can of worms if you're unlucky.

[motornut]

wondering if the shoes are reaching/touching the drums?
nothing to push the air out.
does the ebrake work?

[cam_114]

I took the bleeder nipple off, cleaned it up... Fluid is flowing from it now.
Going to do the prop. valve reset tomorrow again..

One thing I was curious of,
If I'm doing this test, and i find a clogged or slow dripping line, should the truck be running or at least power on so I can see whether the brake light will go off or not.
Or rather should I just do it with engine off and hope to hear the "click" noise...

Also I read on another forum that driving about 25mph then slamming the brakes can reset the valve, i'm unsure of the logic of this but he swore it works...

Both front brake lines were outputting fluid at a very high level, where the two back ones seemed to be 75% less fluid than the front.

Motornut;

I'm unsure at this point if they are reaching the drum.

Im pretty sure the ebrake is good, forgot it on one day and realized it when the truck wouldn't accelerate in reverse.

[cam_114]

The rear is closer to firewall, and is also the smaller than the front.

[tard01]

Ok, if rear is closest to firewall, you should have a decent amoubt of fluid coming out the rear as tthis means you can't possibly mess up applying pressure to it.

E brake i is a seperate system from brake system and fluid/air.. its mecganical... It can be in or out of whack on its own, irregarfless of the shoe/drum... Ie, if the spring is wprn out or installed wrong.

I would try prop test first.. then replace master cylinder.... Prop test again if needed... Then if still low pressure, you might want to see if you can individually seperate the rear line and flush it in segments... Then prop valve as a last resort... That's the money saving plan anyway.
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[cam_114]

If I replace the master cylinder, would it be just as good to purchase a rebuild kit or a brand new cylinder with piston, seals, etc.

[tard01]

A rebuilt master cylinder is only $20.. and new master cylinder have fail rates in them, so I'd just buy a rebuilt one for $20.. just cause the cost vs frustration is so low
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[motornut]

If the Ebrake still works then your close in ajustment
if it wasn't, I would check the shoes are ajusted,lubed and moveing
need to make sure they are out enough,heck maybe a little tight just to bleed

[cam_114]

I have two options for the master... Purchase rebuild kit, or purchase a remanufactured one...

Not gonna buy new.

A remanufactured one doesn't come with replacement seals etc...
Rebuild kit comes with all that and i'd basically replace all my seals on the current master.

"It's basically a pressure balancing switch, so you need to relieve the pressure on the side that has more.
Open a bleeder on the higher pressured side and push te pedal down until it gets to the floor, it may "click" at that point. It's not always that easy,but it can happen.
If not, close that bleader and "slam" the brake pedal. It's mostly about "speed", hit it quickly,but forcefully. You will not "feel" it with this method though, you will have to check. " - LONGHAIR
http://67-72chevytrucks.com/vboard/s...d.php?t=467025

So i've done the test where I slam the brake pedal, it resets the valve, and turns the brake indicator light off. So at this point I think it's safe to assume there's something going on in the rear lines, or like motor nut mentioned..To make sure it's all greased and set properly.

I've only owned this truck for 6 months, so i'm unsure if the brake fluid has ever been replaced.
Plan of action will be to replace brake fluid, make sure the rear shoes are doing what they should be and hopefully that's all it will need.

Would the master possibly be causing this issue?