84 C10 Brakes

[Ronno6]

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

This is a lot to digest.
First, the bleeding technique.
The technique you cited seems to be exactly opposite every other procedure I have read:
start at the FURTHEST wheel and move progressively to the closest wheel.
In my case, that is" RR,LR,RF,LF.
I have tried 2 man bleeding, which is as you describe, and one man bleeding, using tubing
from the bleeder valve to into a jar with fluid in the bottom and the tubing in the fluid.
Both have yielded the same result.
(This procedure worked perfectly for my F150)
I have held the combo pin open and centering tool in place, although to be fair,
I did neither of those details in my initial bleeding procedures.
But, as the centering tool screws all the way in, I conclude that the valve is centered as it should be.

As for the booster, is there any failure scenario for the booster which would cause rear brakes to skid while fronts do not??
All the booster knows is pedal pressure in, boosted pressure out to the MC.
I do not believe it can differentiate between front and rear.
I may be wrong.....

Yes, both of the master cylinders which I have installed are identical to the original.

[irk_miller]

The scenarios I pointed out in the other thread would give you exactly what you're dealing with.  The pushrod misalignment, no play (1/4" is what Chevy calls for) in the brake pedal which cause loading up on the brakes: or having the lines on the m/c reversed.  I also suggested a possible issue with the check valve, which Henry also suggests.  I really think you've covered everything else in the system, so the issue seems to point to the booster.

[Ronno6]

When I loosen the 2 nuts securing the MC to the booster, the MC does not move.
With the significant spring tension on the MC plunger, I would think that the  MC would move forward when the nuts were
loosened if the pushrod was misaligned.
There is 1/4" or better pre-engagement travel in the brake pedal.
With the MC nuts loosened, I step on the pedal and there is about that 1/4" of pedal travel before I observe the MC move.

I do not know how else to check for pushrod misalignment.

And, it is not possible to swap brake lines at the MC as thread sizes differ.

As for MC to booster compatibility, I have purchased MC's from 2 different sources. They are identical to each other, and the original.

I cannot find the inspection port/air filter in the vacuum unit Henry refers to.
I will see about replacing the check valve.

Thanks.

[bd]

The technique ... I have read:
start at the FURTHEST wheel and move progressively to the closest wheel.
In my case, that is" RR,LR,RF,LF.

This procedure ^^^^^ is correct.  It is done this way to remove the greatest volume of contamination and air in the fewest number of strokes to re-establish clean fluid delivery to the cylinders.

All the booster does is transfer and amplify pedal pressure to the M/C.  It does not differentiate between the front versus the rear brakes.  Hydraulic pressure is administered by the M/C, moderated via the combination valve.  If either pushrod was too long the brakes wouldn't fully release, OR they might apply without driver pedal application as the fluid heats.  Neither symptom has been described.

IMSM, the front and rear brake M/C lines have different sized and threaded fittings to prevent inadvertent swapping of the lines.  It would take a concerted effort to mix them up.

It seems the real issue here is that the front brakes won't lock.  For the moment, concentrate your efforts on the front brakes.

Revisiting the basics, did you BENCH BLEED the M/C before installing it into the vehicle?

[Ronno6]

The technique ... I have read:
start at the FURTHEST wheel and move progressively to the closest wheel.
In my case, that is" RR,LR,RF,LF.

This procedure ^^^^^ is correct.  It is done this way to remove the greatest volume of contamination and air in the fewest number of strokes to re-establish clean fluid delivery to the cylinders.

All the booster does is transfer and amplify pedal pressure to the M/C.  It does not differentiate between the front versus the rear brakes.  Hydraulic pressure is administered by the M/C, moderated via the combination valve.  If either pushrod was too long the brakes wouldn't fully release, OR they might apply without driver pedal application as the fluid heats.  Neither symptom has been described.

IMSM, the front and rear brake M/C lines have different sized and threaded fittings to prevent inadvertent swapping of the lines.  It would take a concerted effort to mix them up.

It seems the real issue here is that the front brakes won't lock.  For the moment, concentrate your efforts on the front brakes.

Revisiting the basics, did you BENCH BLEED the M/C before installing it into the vehicle?

I absolutely DID bench bleed the MC.
Serious volume of fluid comes out of the calipers when I bleed. I cannot believe there is air in the system anywhere.

One note:
When I had my wife depress the pedal with engine running and I was watching the hoses, I did note a slight movement of each caliper . I know they are getting pressure, just not enough??

[bd]

This problem has been so persistent as to seem unique, though probably simple in nature.  Bear with me.  Did you try gravity bleeding of the brakes?

[bd]

Please describe how you bled the M/C.

[Ronno6]

This problem has been so persistent as to seem unique, though probably simple in nature.  Bear with me.  Did you try gravity bleeding of the brakes?

I do not think so...I am not familiar with that method.

[Ronno6]

Please describe how you bled the M/C.

I clamped the MC body in a vise at the flat spots just ahead of the flange.
I removed the reservoir cap, and filled each res about 1/2 full.
I inserted tubing tapered nozzles into the discharge ports, with a piece of clear tubing
attached to each. The tubing looped over the top of the res and were held into the fluid
by a clip that attached to the center web between the 2 res sections.
I slowly depressed the plunger and allowed air to be pushed into the res, then slowly allowed the piston to return to
its at rest position. I repeated this til no more air came out of the ports.
Then, I removed the tubing nozzles and inserted plastic threaded plugs into the ports,
and installed onto the booster.
I even got my wife to help me further bleed on the truck.
Loosen 1 line fitting, have her depress and hold the pedal, then tighten the line fitting.
Repeat for both lines.

[Irish_Alley]

when you put it on the truck and rebleed it did you get air?

[Ronno6]

when you put it on the truck and rebleed it did you get air?

I cannot say for sure, as the fluid was squirting out around the fitting.

ps...replacing the booster check valve did not help.

[bd]

Please describe how you bled the M/C.

I clamped the MC body in a vise at the flat spots just ahead of the flange.
I removed the reservoir cap, and filled each res about 1/2 full.
I inserted tubing tapered nozzles into the discharge ports, with a piece of clear tubing
attached to each. The tubing looped over the top of the res and were held into the fluid
by a clip that attached to the center web between the 2 res sections.
I slowly depressed the plunger and allowed air to be pushed into the res, then slowly allowed the piston to return to
its at rest position. I repeated this til no more air came out of the ports.
Then, I removed the tubing nozzles and inserted plastic threaded plugs into the ports,
and installed onto the booster.
I even got my wife to help me further bleed on the truck.
Loosen 1 line fitting, have her depress and hold the pedal, then tighten the line fitting.
Repeat for both lines.

^^^^ Perfect technique.

Gravity bleed:
Top off the M/C with fresh fluid.
Set the cover in place but do not snap down.
Open the LR bleed valve and let fluid drip into a collection pan for 20-30 minutes while maintaining the M/C fluid reservoir level above half-full.
Close the bleed valve and repeat the process at the RR, then RF, then LF, in turn.

Note that the LR is bled before the RR because the tee fitting in the rear axle brake line is generally mounted toward the right side of the pumpkin.  Otherwise, LR vs RR is arbitrary.

Sometimes, gravity bleeding will purge air that is reluctant to escape.

Is it possible for you to post a pic of either front caliper showing the hose and bleeder valve in relation to the "topside" of the caliper?

[Ronno6]

Thanks,
I'll give it a shot.
I should be able to snap a photo.

[Ronno6]

An interesting observation:

The line fittings at the master cylinder are opposite sizewise to those at the compo valve.
The front. 1/4" line has the larger fitting at the mc, but the smaller at the pv.
Line size has been verified, and the fittings are definitely connected correctly.
Just seems odd...........

One other note.
I tested the pv switch function using a 12v test lamp connected to the + battery terminal.
The light did NOT illuminate when the pedal was pushed with vacuum assist.
Either the poppet does not move or the switch or valve is defective.
Next will be the same test with a bleeder screw opened.

I really do not understand what is going on here.
I clearly am not getting enough pressure to the calipers relative to the pressure going to the rears.
Maybe I will need to invest in a pressure kit:
https://www.amazon.com/KKmoon-Pressure-Tester-Braking-Testing/dp/B01DLW1O26/ref=pd_lpo_vtph_263_bs_tr_t_1?_encoding=UTF8&psc=1&refRID=8QQQRFP9207VHP7VQV03&dpID=514ZNcPPtPL&preST=_SY300_QL70_&dpSrc=detail

[Ronno6]

I performed the gravity bleed this morning.
Good news/bad news:
Good news is that pedal feel is greatly improved. Not nearly so mushy, and pedal stops much higher than before.
Progress...........
As with pedal pressure bleeding, not much fluid comes out the rear brakes, and a bunch comes out the calipers.
But, as it is the rears that are locking up, I don't see air in that end being a problem.


Bad news is that the rear wheels still lock up.
Grrrrrrrrrrrrrrrrrr

Now. I have TRIED to get the prop valve switch to move, by opening a bleeder screw and pressing the pedal with ah 12v test light connected to + batt on one end and the probe connected to the prop valve switch terminal.
Zero,nil,nada. Tried front and rear and never got a light.

Looking like the NEW prop valve is bad??