JB8 DRW Front caliper bleed problem (solved and turned into MC discussion)

[rodek]

Hi,

I had the pass side front brake line drip completely dry while upgrading a stock -99 K2500 Suburban fully functional JD7 brakes to JB8 Dually brakes.
After which I had serious trouble bleeding the system,This caused some frustration (to say the least) since I'm not new to brake systems and bleeding.
There is initial bite and the burb stops OK'ish but pressing harder makes no increase in braking power and the pedal can be pushed all the way to the floor.
Started googling and found out the ABS 4WAL system has a unique bleeding procedure that I luckily was able to cover with my Tech II scanner.

Still, no luck after bleeding and running the ABS procedure many times, still spongy pedal.

Got really frustrated and started pinching rubber hoses shut to possibly isolate the problem.
Rear brakes blocked the pedal is super spongy and there is really little braking power front.
Front brakes blocked pedal is semi ok and the back drum brakes grab ok even locking the wheels.
This makes me to believe the problem is in the front circuit.

Now here comes the weird part and the question:
Either front left side or front right side blocked (and rears also blocked) pedal gets increasingly harder/better.
Both front L&R and back brakes blocked the pedal is normal and hard!!
This makes me to believe that the MC and ABS are 100% functional.

What the heck could cause the front calipers to not bleed properly??
There are no bubbles coming from the bleeders and apparently there is still a lot of air trapped inside, since there is very little sponginess in the brake pedal when the front hoses are pinched (with rear line pinched too) and serious sponginess when the calipers get the full MC pressure.

BTW, calipers are installed properly on R/L sides, bleeding nipple is pointing up.

Calipers are for the JB8 DRW 12x1.5" discs (3.38" bore) as opposed of the original JD7 12x1.3" disc calipers (3.18 bore), rear cylinders are the same 1.1875 bore for 13x3.5" drums.
MC should be the same 1.25" diameter in either setup, only difference is that the DRW has a larger reservoir.
If anything with the larger bore calipers, I'd expect increasingly more brake force the deeper the pedal travels.

 

[454cid]

What are you using for discs and pads? Any chance the fluid is leaking past the seals? DRW discs and rotors are thicker, so if you've still got the SRW parts the pistons will be quite extended compared to normal.

 

[rodek]

Hi!

I got the correct pads for the JB8 DRW front 12x1.5" application, pistons are almost all the way in since everything is brand new.

Oh, and no leaks, all parts bone dry.

 

[Schurkey]

I had the pass side front brake line drip completely dry

Start by bench-bleeding the master cylinder, or at least tip the front of the master "down" so that the air bubble trapped inside will rise to the rear, and escape into the reservoir when you tickle the primary piston--either by tapping the brake pedal, (master cylinder still bolted to the booster) or pushing the primary piston with a wood dowel or Philips screwdriver.

found out the ABS 4WAL system has a unique bleeding procedure that I luckily was able to cover with my Tech II scanner.

Same/similar to the procedure attached below?

started pinching rubber hoses shut to possibly isolate the problem.
Rear brakes blocked the pedal is super spongy and there is really little braking power front.
Front brakes blocked pedal is semi ok and the back drum brakes grab ok even locking the wheels.
This makes me to believe the problem is in the front circuit.

Agreed. But I'd be suspecting the air is in the master cylinder front-brake chamber.

 

[rodek]

Hi, thanks for your reply!

Yes, did bleed the MC and with the MC pointing down, tickled out the few miniscule air bubbles that had entered probably from the time it takes to screw on hard lines. There is a clear fluid gush from both rear and front circuits in the reservoir when the piston passes the internal MC fill holes. ABS was bled by GM service manual instructions with the Tech II running the automated bleed cycle and multiple 2 person caliper bleeds in the correct order for multiple times. Caliper and cylinder bleed fluid is clear and without any bubbles.

Had some time to play with the calipers today, remowed them and had a narrow board between the pads to get the pistons to push out a bit further while bleeding. Also had the calipers loose and bleed screws pointing straight to the sky. Maybe had a few bubbles emerge from the bleeders while bleedin for the 100th time.

Rechecked that when the front brake hoses are shut with brake hose clamps, the pedal is really good, almost no give and the rears are biting hard while test driving like this. So as per my logic there should be no air trapped in the ABS and no fault in the MC, unless the MC has an internal leak between pri / sec side. MC has been changed about 5 years a go.

Many people state that the brakes were bad in the 400 platform to begin with but still...

I can drive the Burb, brakes are good for driving but I'd like to see more max braking power when stepping on the pedal really hard. At least while driving the pads will settle a bit and maybe find a bit more grip. If not I think I'll have to try a new MC and then even calipers up front....

Could anyone confirm that their JB8 setup (3500 dually with 12x1.5" discs and 13x3.5" drums) uses the 1.25" master? That is my only concern even though I have triple checkd GM literature, GM parts catalogs and finders and Rockauto searches.

 

[rodek]

OK so Finally got the bakes bled, as stated, with _multiple_ bleed cycles around the truck and Automated ABS bleed procedures etc.
Really difficult system to bleed and I can say this is not my fist time bleeding.....

Maybe the final bubbles came out after I remowed the calipers and had the pistons come out a lot with narrow boards between the pads. Sort of simulating a situation where the pads and rotors had worn already considerably.
Also had the bleed screews at the highest point vertically and horizontally. After driving to the yearly inspection the brakes wore in a bit and now they seem OK and actually performed well in the brake dyno at the inspection.

To the MC discussion...
I've read quite many threads regarding the JXX > JB8 brake upgrade and 100% understand the brake system details.

JD7 Hydrobooster gives so much force that you loose the pedal "feel" after half stoke of brake pedal travel and the pedal travel is long because of the C3500 biggest bore calipers 3.38" (JB8 DRW) need so much volume from the MC.
All MC's in C/K2500 and C/K3500 are 1.25" (31,75mm) for the big brake setups with disck / drum during the late 90s.
The NBS MC's are either 34mm (1.34") or 37mm (1.46") depending on tonnage and brake setup.
The 2001 1500's got the 34mm MC for example, later 2500's got the 37mm MC etc..

Since there is no problem with brake boosting, there is force and pressure available with the hydrobooster, I'd like to try the 34mm or even the 37mm NBS MC.
This is to get the pedal travel shorter while maintaining the max force via hydrobooster's brute force.
Naturally in an emergency, if for some reason there is no boosting available, the 34mm MC need more leg force... (and 37mm esecially!)

The 34mm MC gives about 14% more volume per any amount of stroke, and the 37mm gives 35% more volume than the 1.25" stock MC.
My thinking is that trying the 34mm should be a no brainer and even the 37mm could be ok with the question of any possible loss of boosting in panic stop situation.

Oh, I'd need to change the hydrobooster to NBS with the NBS MC since the NBS master cylinder whas an outer diameter of 1.8" that will NOT fit into the JD7 hydroboosters 1.6" hole .
There is a good thread about using a NBC Hydrobooster fornt half with the original HB back half to solve the issue.
(Changing HB's directly is difficult because of different attachment pattern in the firewall and push rod length)
see: https://www.gmt400.com/threads/1995-k3500-cc-rv-puller.67684/

 

[GoToGuy]

If the engine is running and power steer pump is working you have constant brake assist pressure. As per the handbook and service manual a loss of normal brake pressure assist, in an emergency or critical situation the hydro booster accumulator retains pressure for two brake applications . Reality , apply constant braking till complete stop.
You say emergency braking. But are saying in an " immediate maximum braking stop" or as an " emergency loss of hydro boost assist stopping " . Those are an apple and orange problem .
The hydro boost normal operation is not like vacuum assist braking. It has different feel , different learned muscle memory. Ensure the rear shoes are properly adjusted , being out of adjustment means more pedal travel before shoe braking. Good luck.

 

[rodek]

Hi

Yeah good point, I actually was meaning a situation where I'd completely lose the hyd. power assist and even had the nitrogen reserve depleted (emergency loss of hydro boost assist stopping)
Tried it a few times while moving the Suburban downhill in the yard without engine power, it needs some serious leg force for braking.

The whole thought about changing to a bigger bore master is that in the other discussed situation (immediate maximum braking stop), I'd like to feel the pedal reaching the maximum braking force before almost hitting the floor.
I drive european vehicles in general on a daily basis, so the mushy brake pedal really sorta feels wrong but naturally this is also partly because of the HB as you stated.

I've got 35" wheels and "empty" weight of 8000lbs so I need "all the brakes" I can get.
Rear brakes (JB8 setup) are properly adjusted.

Thanks!