87 R30 - Weak Rear Brakes

[HDBobbers]

I'm having some issues with my brake system and need some outside perspective. This is on a 1987 Chevy R30 dually, disc/drum setup.

Problem is that i don't have any braking power at the rear. I can actuate the wheel cylinder. The cylinder will move, but only slightly.

What i've replaced in the system so far:

Master cylinder (bench bled prior to install)
Proportioning valve
Front lines
Front hoses
Front calipers
Rear lines
Rear T block (load sensing valve removed)
Rear wheel cylinders
Rear brake shoes
Rear drum components

- Popped the pigtail off of the prop valve and inserted stabilizing bolt

- System was pressurized to 10psi stable with a Motive Power Bleeder

- Bleeder barrel was filled and pressurized to 15psi and bled rear left (furthest, line runs on right frame rail), rear right, front right, front left

- Shoe adjusters dialed out for full contact with drum, then backed in slightly

At this point, I put the axle back together and started the vehicle, put it in gear, and let the rear turn (truck is on jack stands). Depressed the brakes and they are soft, though they have always been soft since i bought the truck. The rear continues to turn.

Took the rear apart, started the truck, and pressed the pedal with a long board while watching the rear brake system. I can see the wheel cylinder moving... but only slightly.

I'll mention that if i press the brakes to the floor, i am met with some odd constant jittering and what sounds like power steering screech. Could this be a failed brake booster? Aside from the rear brakes not actuating correctly, the only thing i can think of that would cause some sort of hydraulic screech or throwback on the pedal would be the brake booster, though I have never needed to replace one.

Anyone have some thoughts?

 

[Keith Seymore]

I see from the brake system release chart that the '87 R30 has the rear height sensing valve.

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That's that Rube Goldberg funky linkage thing above the rear diff.

Have you verified that it is working properly (or, ideally, removed it)?


K

 

[Keith Seymore]

Took the rear apart, started the truck, and pressed the pedal with a long board while watching the rear brake system. I can see the wheel cylinder moving... but only slightly.

Be careful applying the brakes with the rear drums removed; if everything is working as it should be the rear wheel cylinder pins will come flying out.

K

 

[HDBobbers]

Yeah... rear load sensor is gone. Replaced with a T block to route left and right. So, that's not a can of worms that i need to get into, heh.

Understood on actuating the brakes without the drum. I needed to see if the wheel cylinders were actually moving. Which they are... just not much. Barely discernable.

 

[1987 GMC Jimmy]

That hydroboost issue sounds wonky. Is it harder to press the brakes, and have you noticed any PS fluid leaks? Is your pump belt okay, or have you heard any groaning or whining from the pump?

 

[chengny]

I'll mention that if i press the brakes to the floor, i am met with some odd constant jittering and what sounds like power steering screech. Could this be a failed brake booster? Aside from the rear brakes not actuating correctly, the only thing i can think of that would cause some sort of hydraulic screech or throwback on the pedal would be the brake booster, though I have never needed to replace one.

Anyone have some thoughts?

Here are some thoughts - always consider the source:

Whether or not there is a problem with the hydraulic power assist (Hydroboost) system is irrelevant at this point - you have a fundamental issue with the braking system's rear main hydraulic circuit.

Power assist brakes are provided to do just that - "assist". The brakes are designed to be fully functional under any operating conditions - even with a total loss of the power assist system. Application of full braking power (without power assist) may require more effort on the part of the operator, but that additional force is nothing a healthy adult male can't easily produce with his leg muscles.

The point is, the lack of pressure/flow to the rear hydraulic circuit - that you are apparently experiencing - should not be dependent on power assist function - and should be resolved before investigating any issues with the Hydroboost system.

From what I see above, it appears you have essentially installed a complete new hydraulic braking system. Working on the assumptions that: there are no defective parts, all the parts are for the proper application and they were installed correctly...the next logical checks would be the operational input/output components.

As far as output components, you have completely rebuilt the rear drum assemblies and replaced the actual drums. You have verified that the shoes are adjusted properly and have minimal clearance to the drum surfaces. With the drums removed and the brakes are applied, the wheel cylinder pushrods are observed to move - but only to a "barely discernable" degree. So, it appears that the rear wheel cylinder pistons are being stroked - but not enough to force the shoes against the drums for normal braking.

That leaves the input components. See the first part of your opening sentence in the quote above:

I'll mention that if i press the brakes to the floor,

First, if you can depress the pedal to the floor, you do not have sufficient "pedal reserve". That is just a term for the distance differential between the brake pedal and the floor board - from when the brakes are at rest and when they are fully and forcefully applied. GM is looking for a minimum of 75% pedal reserve - with the brakes stomped on:

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The usual causes for lack of proper pedal reserve are a bad MC, air/leakage in the hydraulic system, excessively worn/ out of adjustment friction components, etc. But you have replaced all that stuff.

Another common cause of low pedal is installation of the wrong MC or wrong pushrod. From GM:

New or Rebuilt Master Cylinder


- Always closely compare the new master-cylinder with the old master-
cylinder. Minor variations in the size and depth of the primary piston can
result in significant reductions in braking performance.

- Examine the end of the master-cylinder where the pushrod contacts the
piston. The depth and diameters of the pistons should match.


Another reason for low pedal is that the MC piston is not being fully stroked. Under normal conditions, when the piston moves forward, it first pressurises the primary (front) hydraulic circuit. The secondary circuit is pressurised only after the piston moves past the primary outlet port. If the pushrod motion is stopped before the piston reaches full stroke, the secondary doesn't get full pressure/volume and the wheel cylinder pistons don't move as designed.

Since you have always had a soft/low pedal (and you have checked/replaced everything else), I would look closely at the length of the MC pushrod and check the brake pedal mounting/linkage for worn/loose components.

GM sells an adjustable MC input pushrod to accommodate modified installations:

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P/N 18060010 or 5469384


http://www.ebay.com/itm/GM-18060010-Rod-Kit-5469384-/270698681845

 

[HDBobbers]

Chengny - MUCH appreciated for your input. I didn't even think to check the pushrod. I **** you not, i just took apart the front of the hydro and the previous owner had slapped a rounded over bent bolt with a washer in there.

I just ordered the new pushrod.

Thank you for the input. I'll update when i receive the parts and install. Until then... time to ride the Harley.

 

[HDBobbers]

So, an update. GM p/n 18060010 / 5469384 looks to be for a pancake vacuum diaphragm booster. I should have mentioned, or dug a bit deeper, that this is a hydroboost application. I'd be looking at item 29 (pn 00377397 / 129959). 18060010 / 5469384 looks to be about an inch too short for the application.

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REGARDLESS... it was good input. For the time being, i modified the new output rod with a longer length adjusting bolt. Works for the moment and gave me my pedal back after the rod was adjusted out based on the measurements of booster cavity depth, rod length, MC neck length, and MC cavity depth. That said, my pedal is now hard as a rock.

So, i'm back to suspecting that something is wrong with the booster unit. I'm going to pull it tonight.

 

[chengny]

Hydroboost

Here are some good cheat sheets on Hydroboost brakes that might be of some help:

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