84 C10 Brakes

[Ronno6]

the brake light should illuminate when you first turn the key "on" or if you ground out the wire that connects to the P valve

Yup. Nope...no brake lamp on the instrument cluster is lighting........
I wired the switch connector directly to the neg battery terminal....nuttin.
I removed the fan shroud again to gain access to the prop valve.
I unsctrewed the brake light switch and inserted the bleeder tool.
The tool threaded all the way in, so I conclude that the valve is still centered. Whew!
With the tool installed, I re-bled the brakes....again. (not redundant as I had already bled and re-bled the system.)
No change to the function of the system.

When depressing the pedal with engine running, it meets good resistance high up, but will
go way down rather slowly.
I am still thinking master cylinder....again.

I think I'll call this truck the "againmobile."

[Ronno6]

I have seen posted on another forum that the JB5 calipers have a larger, 2.94: bore size, but have not been able to find out the bore size of the JB3 calipers.
The larger caliper bores may mean a delay in application.
But, the JB5 rear cylinders have a SMALLER bore than the JB3
One would think that the JB5 would activate sooner than the JB3 rears.

How do the 2 stages of the MC work?
Does the first stage send fluid to front and rear, or just one or the other??

I dunno. Maybe I need to back off on the parking brake some more............

[Ronno6]

Another new MC,
Another brake bleed (with the centering tool in the prop valve this time)
Another parking brake adjustment, and


NO CHANGE...........

About all that  is left is to revert the front brakes back to JB3 or upgrade rears to JB5.
Barring any other revelations.........

According to NAPA, the caliper piston bores are the same between JB3 and JB5 calipers.
I fail to see where that can cause an imbalance.

[irk_miller]

Which mc are you running?  Going back through this thread like 6 times. 'againmobile' . You said you changed the mc.  If the mc bore is too small, you'll get a soft pedal and grabby brakes. (you say it locks up sometime)  If the mc bore is too big, you'll get a hard pedal and less caliper pressure.

[Ronno6]

Which mc are you running?  Going back through this thread like 6 times. 'againmobile' . You said you changed the mc.  If the mc bore is too small, you'll get a soft pedal and grabby brakes. (you say it locks up sometime)  If the mc bore is too big, you'll get a hard pedal and less caliper pressure.

From what I have found the master cylinder is the same for either JB3 or JB5
Some are listed as fitting "all US trucks with vacuum power brakes.

[Ronno6]

I observed the 3 brake hoses in the system while my wife depressed the pedal w/motor running and could detect no lengthening or swelling in any of the hoses.

I am almost at a dead end here.
SOMETHING is going on here and I don;t know what.......

The only option I can try at this time is to switch rear cylinders to the smaller bore units for the JB5 system.
They will reduce the force applied to the shoes by 17%
Mavbe the lower volume will reduce pedal travel and increase pressure to the calipers.
That is an inexpensive swap, and, if that doesn't cure the problem, at lease I will have then already if I upgrade the rears to JB5.
I can still find no difference in bore size between JB3 and JB5 calipers.
Maybe the JB5 calipers have an internal pressure regulator in order to moderate the increased pressure from the double diaphragm brake booster ??

I am trying not to overthink this, but none of the simple solutions have worked.....

[Ronno6]

So, I tired AGAIN, this time blocking the pin open on the end of the prop valve, inserting the centering pin, and bleeding the front brakes....AGAIN.

And, AGAIN, no improvement.
I have ordered the 15/16" JB5 rear cylinders.
My last hope without major changes.

[bd]

Have you verified that the rear shoes are properly sized to the drums?

[Henry]

Hi Ronno6:
Another basic to check:
1. Recheck to make sure you did not install the combo valve backwards...on my truck it is possible to install it backwards...if this is possible on yours, the manufacturer should have made some "in" and "out" indications on the ports.

What is the status of your vacuum power brake unit?... has it been replaced or is it still the original one before the problems started? For my old truck, there were at least 4 different vacuum units available and I am wondering if maybe there is an incompatibility of your vacuum unit and the new master cylinder...or maybe the master cylinder push rod that goes between the two is not adjusted correctly, if it is adjustable. Some have an adjustable crown nut at the end of the rod. There are other things that can go wrong with the internal valving and seals of the vacuum unit to cause either hard pedal or soft pedal feel.

Regards,
Henry

[Ronno6]

Have you verified that the rear shoes are properly sized to the drums?

Yes, both are 11" dia x 2" wide

[Ronno6]

Hi Ronno6:
Another basic to check:
1. Recheck to make sure you did not install the combo valve backwards...on my truck it is possible to install it backwards...if this is possible on yours, the manufacturer should have made some "in" and "out" indications on the ports.

What is the status of your vacuum power brake unit?... has it been replaced or is it still the original one before the problems started? For my old truck, there were at least 4 different vacuum units available and I am wondering if maybe there is an incompatibility of your vacuum unit and the new master cylinder...or maybe the master cylinder push rod that goes between the two is not adjusted correctly, if it is adjustable. Some have an adjustable crown nut at the end of the rod. There are other things that can go wrong with the internal valving and seals of the vacuum unit to cause either hard pedal or soft pedal feel.

Regards,
Henry

Thanks for that.
The combo valve cannot be installed backwards.
The fittings are different sises.
Vacuum unit has not been changed. It works and holds vacuum after engine has shut off.
I did not see any adjustment capabilities.

[Irish_Alley]

curious but did you do any work to the brake shoes? have you verified everything is installed right and the primary shoes (front) is actually the smaller shoe?

[Ronno6]

curious but did you do any work to the brake shoes? have you verified everything is installed right and the primary shoes (front) is actually the smaller shoe?

Brake shoes are new.
Primary shoe (shorter lining) is installed to the front.

[irk_miller]

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.

[big_al273]

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?