Brake issue, Hydro boost and rear disc

[davkenrem]

Couple of years ago I did a Hydroboost swap concurrant with installing a 9.5" SF rearend with disc brakes on my 1997 K1500 Suburban.
I used an axle from a 97 K1500 Z71 pickup. The Hydroboost was harvested from a 2002 Z71 Suburban as were the dual piston rear brakes and 13 in rotors.

Two weeks ago I jumped into the truck and headed out and my pedal goes to the floor. I assumed my front pads were shot, parked it and and finally got around to looking at it.

To my suprise all the pads had plenty of material. I was slightly low on fluid so I hooked up the pressure bleeder and looked for a leak. Could not find a leak anywhere. Tried to bleed brakes, starting at the rear, opened bleed screws, no fluid either side. Goto front plenty of fluid coming from both. Still no pedal.

A mechanic friend suggested a leak between the master cylinder and the hydro boost. Pulled master cylinder. No leak I could detect. He also speculated that the rear discs may have never worked since the Kelsy ABS was not proportioned for them. Hard braking in the truck did seem biased to the front.

Wondering if the ABS is culprit?

Anyone done a Hydroboost swap on a 97 with rear discs?

What did you do for proportioning?

I'm going to put on another master cylinder and see if that is it, could be. But I will still have the proportioning issue.

Is an ABS delete a viable option?

Could use some advise.

Thank you in advance.

Dave

 

[Schurkey]

Verify caliper pistons not sticky or seized, caliper mounts not sticky or seized.

Inspect fluid flow just ahead of rear brake hose. Common for hoses to plug. Move forward as needed until you have fluid flow.

Connect scan tool, perform ABS auto-bleed procedure, then bleed brakes as normal.

 

[Dropped88]

Crack hardline loose at rear brake hose and make sure hose hasn't collapsed

 

[davkenrem]

Resurecting an old thread. Over the last year have had much time to work on the Burb. Lost my Father in law, Mother and cousin and had to deal with all three estates. Needless to say I never got back on the brakes. I did replace the Hydroboost and master cylinder with new ones from a 1997 K2500 Suburban. The difference here was the master cylinder has two bowls for fluid, a large one for the front and a small one fro the rear, whereas the One I orginally sourced from a 2002 1500 Z71 Suburban has one that has one large bowl for both front and rear. (See pics below)

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2002 1500 Z71 Suburban 1997 K2500 Suburban


So I guess my main question is, Has anyone a rear disc brake swap on a GMT400 96-99 with ABS and gotten the proportioning to the rear brakes to work correctly.

Seems like with the ABS proportioned for rear drums the discs will never have enough pressure.

Anyone have any thoughts.

 

[Schurkey]

Do discs actually need more pressure than drums? Not so far as I know, but I'm prepared to learn something.

I'm thinking the pressure is fine, IF (big IF) your hydraulic system is working properly. Perhaps your rear-disc caliper pistons are too small. More surface area on the (bigger) piston(s) would make for additional force on the pads.

Caliper piston area has to be balanced against master-cylinder piston area, to keep pedal height vs. pedal force reasonable.

Disc brakes generally get paired with a large fluid reservoir, because unlike most drum brakes, disc brake pad wear requires the pistons to move progressively farther out of the bore--which means more fluid trapped in the caliper. The reservoir has to be big enough so that the pads can go from "new" to "worn out" without emptying the reservoir.

This reservoir

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is divided internally so that a fluid leak in one circuit doesn't empty the reservoir for the other circuit--you'd still have some braking ability.

 

[Erik the Awful]

Do discs actually need more pressure than drums? Not so far as I know, but I'm prepared to learn something.

Yes, they do. Drum brakes self-actuate, meaning that once they bite, their geometry causes them to multiply your effort. The wheel cylinder also has a significantly smaller piston diameter than disc brake calipers. That's part of why it's a bad idea to swap newer master cylinders willy-nilly. You need to calculate the piston areas and size your master cylinder appropriately. Disc brakes require more fluid volume and more pressure, which is why disc brakes require a booster when old 4-wheel drum systems got away with no booster.

 

[davkenrem]

I understand what your saying, but is there a correct way to modify this brake system to keep the ABS and use Disk in the rear? Has anyone successfully accomplished this?

 

[Erik the Awful]

If you want to swap to newer brakes without doing a lot of math, you'll want to swap the system as a whole. I haven't done it.

 

[Schurkey]

Drum brakes self-actuate, meaning that once they bite, their geometry causes them to multiply your effort.

True. Even the leading/trailing shoe drums have some amount of self-actuation.

The wheel cylinder also has a significantly smaller piston diameter than disc brake calipers.

Drum wheel cylinder piston ~1 inch diameter = .7854 square inches
Disc caliper piston ~3 inches diameter = 7.0686 square inches.
Disc calipers provide something like NINE TIMES the force on the friction material, with the same fluid pressure, depending on the vehicles in question. This somewhat compensates for self-actuation of the drum system.

it's a bad idea to swap newer master cylinders willy-nilly. You need to calculate the piston areas and size your master cylinder appropriately.

I agree with this.

Disc brakes require more fluid volume and more pressure, which is why disc brakes require a booster when old 4-wheel drum systems got away with no booster.

Given that disc/drum systems use the same size master cylinder pistons for both the disc and the drum parts, I have trouble believing that discs need more volume. Low-drag calipers certainly need more volume, but regular calipers?
Drum pistons are small-diameter, but move a larger distance.
(Non-low-drag) Disc caliper pistons are large diameter, but move a very small distance. I'm not prepared to work the math, but I bet fluid volume for regular (not low-drag) calipers is very similar to drum wheel cylinders.

What hasn't been mentioned is the additional surface area of the drum friction lining compared to disc pads. The advantage goes to the drum system.

 

[Erik the Awful]

Disc calipers provide something like NINE TIMES the force on the friction material, with the same fluid pressure, depending on the vehicles in question. This somewhat compensates for self-actuation of the drum system.

That's backwards. Disc brakes have nine times the piston area, but that translates to 1/9th the force.

Given that disc/drum systems use the same size master cylinder pistons for both the disc and the drum parts, I have trouble believing that discs need more volume. Low-drag calipers certainly need more volume, but regular calipers?

Remember, the caliper piston has 9x the surface area. They need more volume and pressure. Drum brakes are capable of more braking force, but only when well-maintained. When they're out of adjustment disc brakes are superior.