84 C10 Brakes

[irk_miller]

Can you confirm, for sure, that you put the the brake lines on the right side of the reservoir when you replaced the m/c? If they're swapped, it will cause the rear to lock up.  The reservoirs are two sizes because of the different pressures needed for each system.

Also, the booster pushrod being misaligned will cause locking up since it preloads the m/c.  I can imagine it's possible to preload the rear more than the front since it's first in the m/c to the booster.

[Ronno6]

I just cannot understand why I cannot get my rear brakes to quit locking up under normal braking loads.

Are the adjusters in the drums over tightened?

I have tried both methods of initial adjustment: tightening til drum is locked, then backing off 12 notches, as well as adjusting so that there is no initial contact between drum and shoes.
Neither method has cured the problem.
Shoes,drums and wheel cylinders are all new.

[Ronno6]

Can you confirm, for sure, that you put the the brake lines on the right side of the reservoir when you replaced the m/c? If they're swapped, it will cause the rear to lock up.  The reservoirs are two sizes because of the different pressures needed for each system.

Also, the booster pushrod being misaligned will cause locking up since it preloads the m/c.  I can imagine it's possible to preload the rear more than the front since it's first in the m/c to the booster.

I put the lines on the new MC in the same orientation as when they came off the old one.
But, in fairness, as the problem existed with the old MC, it could be that they are reversed.
The lines coming off the MC are spiraled,tho, and I don't know if they can be swapped without really making a tangled mess.
I will examine to see if they are correct down to the prop valve.

For sake of clarification, which line should go to front and which to rear?

Front line is connected to rear brakes.

I have notice during bleeding that the front brakes get way more fluid than the rear brakes.
Is that proper?

I cannot think that the pushrod is misaligned.
The MC goes onto the booster without encountering any interference.
Good thought,tho........

[Ronno6]

Brakes are hard as rocks without vacuum boost, but, when engine is
running, the pedal goes way down, far past the stopping point when engine is off.

Little to no vacuum = hard pedal.  Leak in the vacuum booster or faulty check valve can put air in the mc and cause a soft pedal.

The booster holds vacuum seemingly indefinitely after the engine is turned off.
Would that indicate that there is no leak?
Isn't the pushrod a solid rod all the way from pedal to MC ??

[irk_miller]

On any disc/drum setup, the big reservoir is for the front (disc) brakes.  Another thing to check is free play in the brake pedal.  There's should be 1/4" of free play, or it preloads the m/c.

[irk_miller]

I guess we have this conversation in two places.  I posted about the possibility of pedal play being an issue in the other string.  SHould be 1/4" or it preloads the m/c.  The rod must meet it's "at rest" position.

[Ronno6]

On any disc/drum setup, the big reservoir is for the front (disc) brakes.  Another thing to check is free play in the brake pedal.  There's should be 1/4" of free play, or it preloads the m/c.

I just reviewed the plumbing.
Front chamber connected to the rear brakes.

I will check pedal free play.

thanks...

[Ronno6]

I guess we have this conversation in two places.  I posted about the possibility of pedal play being an issue in the other string.  SHould be 1/4" or it preloads the m/c.  The rod must meet it's "at rest" position.

Yes, though that was not my intent.
This thread was intended to deal with the system problems;
the other was to discuss the rear upgrade to JB5.

[Irish_Alley]

if the front is connected to the rear this will throw it all off since disk use 90 psi and drums use 40psi

[Ronno6]

if the front is connected to the rear this will throw it all off since disk use 90 psi and drums use 40psi

The brake lines and fittings are different sizes.
The orifices in the MC are different alwo: Larger front for the 1/4" rear line, and smaller rear, for the 3/16" front line.
It is not possible to connect incorrectly......I just tried.
Unless someone has replaced the nuts with incorrect ones......

[Ronno6]

Going to the smaller wheel cylinders did not correct the problem.
I guess the front brakes are just not getting enough  pressure.
They sure put out a lot of fluid when I bleed them.
I cannot imagine that the new prop valve is bad.
All that is left to consider is that I am using the JB5 rear cylinders w/the 2" brakes, and they are locking
up before the proportioning valve increases pressure to the discs.
Possibly the larger 2 3/4" shoes would provide that.
But, as of yet I have not found the backing plates to upgrade the rear system.

Or, maybe I should replace the hoses just because??

I really wish I knew if this problem was present with former owners.
SOMEBODY had to have changed the fronts to JB5.......
I dunno. 
I am at wits end.
Maybe time to admit defeat and take it in to the man..........
That's NOT why I bought this truck.....

[Irish_Alley]

you have the plastic master cylinder dont you? with the smaller chamber in the front? if so then yes rear is in the front

[Irish_Alley]

i was trying to figure out how the lines got switched up since theyre different sizes/nuts and are pretty much not bendable (too far from factory)

[Ronno6]

you have the plastic master cylinder dont you? with the smaller chamber in the front? if so then yes rear is in the front

That is correct.
Most places specify the same part for either JB3 or JB5

[Henry]

Hi Ronno6:

Manual bleeding technique verification:
1. Bleed starting with the valve closest to the mc and work your way to the furthest from the mc.
2. Combo valve held open during all bleeding.
3. Bleeder hose must remain submerged in a container of brake fluid while bleeding.
4. When bleeding, pump pedal up and pressure held before each opening of valve.
5. Open bleed valves only 3/4 turn and must be shut tightly before pedal reaches end of travel.

Concerning the vacuum power unit, there are various part numbers for the Chevy trucks but they all pretty much have the basic same design. The brake pedal pushrod inputs your braking desires and a separate master cylinder pushrod takes the command and applies it to the master cylinder with greater force. Between the two pushrods is air valving and springs which makes all the force multiplication to happen. Even if the unit holds vacuum, there still may be something going wrong inside. Rebuilding one of these things can be done but it requires some special tools and gauge or two. Maybe you can call around and see if there is a shop that specializes in the repair of these things and you can remove it and take it to them for testing just to verify it is OK. I also would encourage you to have someone verify that your new master cylinder is compatible with this older vacuum unit. Even before you do all this, a quick and inexpensive fix would be to replace the check valve on the outside of the unit and see if this makes a difference. At the firewall side of the vacuum unit, there should be a rubber boot covering the brake pushrod interface...just inside this boot should be an air filter...if you take the unit off you can inspect this filter.

You have done everything I would have done and everything is new except the vacuum unit...so that is why I suspect it...is your new mc exactly like your old mc?

Regards,
Henry