Rear disc brake problem

[GMC Guy]

finally got everything bolted back together today and was going to start bleeding the lines. the fronts are pushing fluid no problem, but i can't get any fluid out of the rears? bleeder screw open, nothing coming out.

Master cylinder: http://www.summitracing.com/parts/rsd-dbmc04
Proportioning valve: http://www.summitracing.com/parts/rsd-pv72
Brake booster: http://www.summitracing.com/parts/tff-2232nb
Stainless braided lines: http://www.summitracing.com/parts/rus-672310
Rear Disc kit from Blackbirds

[wyattglock]

Do you have the calipers oriented the right way on the disc? I have put them on upside down before and got no fluid through them.

[GMC Guy]

Originally Posted by wyattglock

Do you have the calipers oriented the right way on the disc? I have put them on upside down before and got no fluid through them.

yep. with the bleeders pointing up

[mrhino]

I had the same problem on a 71 I did a couple of years ago. by chance have you changes the M/c?

[mrhino]

Oh I now see that you have. was your truck a manual brake before?

[Oilbrnr]

Did you bench bleed the MC first? If so, then the next thing I'd look at is the proportioning valve.

[Sabaka454]

So you converted it to rear disc? Did you replace the master and replace the combination valve with a proportioning valve? After you bench bleed the master you can bleed the new proportioning valve with a syringe or hand held vacuum pump as well before installing. If you know anyone with a vacuum pump it make life pretty easy but you'll still most likely need an assistant and have at it the old fashioned way.

[GMC Guy]

Originally Posted by mrhino

Oh I now see that you have. was your truck a manual brake before?

factory power brakes

Originally Posted by Oilbrnr

Did you bench bleed the MC first? If so, then the next thing I'd look at is the proportioning valve.

bench bled it twice now

Originally Posted by Sabaka454

So you converted it to rear disc? Did you replace the master and replace the combination valve with a proportioning valve? After you bench bleed the master you can bleed the new proportioning valve with a syringe or hand held vacuum pump as well before installing. If you know anyone with a vacuum pump it make life pretty easy but you'll still most likely need an assistant and have at it the old fashioned way.

master is for front & rear disc, as is the proportioning valve. the links above are to the parts i've installed. only thing not up there is a rear stainless braided kit from ORD (off road design) that runs from the T on the axle cover over to the calipers.

[GMC Guy]

going to try this procedure today as soon as i can get an assistant.
http://www.73-87.com/7387garage/chas...sion/bleed.htm

[GMC Guy]

still not getting any pressure from the rear of the master. plenty of pressure from the front, but next to nothing from the back. pretty lost at this point....

[mrolds88]

Go through the system from front to back. Start at the M/C. Loosen the fitting. Press brake. Got fluid? Move downstream. Hit every fitting until no fluid or you get fluid from the caliper.

[GMC Guy]

put the disc/drum MC back on just to see what happens. got a little fluid out of the bleeders in the rear. no pressure behind it though, just kinda dribbled out of the hose i had connected to the bleeder.

Put the disc/disc MC back on, annnd nothing. how much pressure should i expect from the rear fitting of the MC? i was having a friend press and hold the pedal while i loosened the fitting to bleed it the rest of the way. The fluid from the front fitting had a good amount of psi behind it. but the rear, just a dribble...
btw, i have bench bled the MC multiple times, still no rear pressure.

Pretty well over this hunk of metal for today, we'll see what tomorrow brings...

[Wrecked1982]

when you bench test i assume front and rear work fine. maybe blow some compressed air into the rear brake line with bleeder open?

[rs74]

Originally Posted by mrolds88

Go through the system from front to back. Start at the M/C. Loosen the fitting. Press brake. Got fluid? Move downstream. Hit every fitting until no fluid or you get fluid from the caliper.

This is what I was going to suggest also.

[donut]

Is there any button or relief valve on the prop valve?
I seem to remember some procedure that held a button on the prop valve to allow fluid to pass through it. This is memory, some searching should turn something up.

This would be after doing what mrolds88 suggested. (what I would do, find out what point fluid stops and go from there.)

[meatwagon83]

is the t fitting on the diff plugged?

[GMC Guy]

so here's what i did, separated the hardline from the braided line above the rear-end and blew compressed air through the open bleeders. re-connected and commenced bleeding. what do ya know? fluid and bubbles start coming from the hose into the bottle! went around and bled all four corners thoroughly multiple times. still don't have a hard pedal, but getting there.
HOWEVER, with the engine running the pedal has no resistance... any suggestions for this problem? the booster is brand new.

[GMC Guy]

i'm going to order this guy and see what we can do. it pushes fluid from the bleeder up to the master, getting all those stubborn air bubbles out.
http://www.summitracing.com/parts/psl-2004

[hatzie]

You can build a top down pressure bleeder from a garden sprayer or just buy one. I use one to flush the brakes every 2 years or so on everything I own or maintain. I built one and I use Motive MC adapters It works great.


DIY
Bleeder http://www.bmw-m.net/TechProc/bleeder.htm
MC Adapter http://www.amazon.com/Power-Bleeder-Adapter-Alumin...

BUY
http://www.motiveproducts.com/?gclid...FQyg4Aod6HkASQ
http://www.amazon.com/Motive-0250-Universal-Brake-...

[GMC Guy]

so today i end up taking the truck in to the shop i always go to for tires, brakes, alignments, etc. and they bled the brakes with their professional grade stuff and got a rock hard pedal. until they start the motor and the pedal is once again super soft....they can't even figure out what the issue is at this point.
I just wanted better stopping power...what have i done??

[INSIDIOUS '86]

I believe the brake line size needs to be smaller for a rear disk so more fluid moves as the pedal is pressed. Drums don't need so much fluid to move to deploy the shoes.
Drum brakes in actuality can stop you way better...once then they fade pretty quick
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[mrhino]

it sounds just like the problem I had with a 71 I converted to rear disc brakes. Thats why I asked about you having manual disc. Once you said you didn't I didnt think twice about it. Since you having the same symptoms I was. I would check your brake pedal ratio. my truck was manual and when I switched to the 4 wheel disc M/c just like the one you have, the ratio was off. it wasn't pushing the brake rod in far enough to push enough fluid to the rear. plus it would engage the safety feature in the prop-valve. I was constantly having to reset the prop valve. Like you when I would bleed the rear i would get fluid out and air out. but when we would start the truck, boom pedal went to ****. I even replaced the master thinking that was the problem. Anyway that was my problem and it took me awhile to figure that one out. crawl under your dash and and just watch the brake pedal movement and see if you have enough travel. if you dont you may have to redrill a new brake rod hole in the pedal.

[GMC Guy]

well i know for a fact that the rears are getting good pressure, i had to drive the truck clear across town to get to the shop. and before i got a feel for the pedal, i could lock the brakes up with too much pedal. plus when the rear was set up on jack stands and the brakes assembled, i started the truck and put it in gear just to check the operation of the rear brakes. everything functioned beautifully and the rotors stopped spinning without a whole lot of pedal. so basically i know the truck will stop fine, i just can't keep a hard pedal with the truck running....

[wyattglock]

One thing I noticed when I swapped to rear disc with my dually was the softer pedal. It stops fine, actually a little better IMHO. The kit I got was from www.diy4x.com, it came with a 1/4" spacer that fits between the caliper piston and the pad. This gave a lot of pedal firmness back after the swap. At first I had installed the caliper without the spacer, but the pedal would travel almost to its limit before stopping the truck. Now it is very comparable to the factory setup after I installed the spacer. I don't know if your kit came with that, but it might be worth a shot.

[v30crewcab]

if the pedal is soft it usually means the master is too small. might need to go up in size a little bit.