Created a problem. rear duramax brakes, stock master

[camarossguy2]

I found out one of my spindles on my dana 70 axle was fubar, and after much looking I finally gave up on a finding the right 14bolt and found a another dana from a 2004 duramax powered 3500 express.
(long story, but i highly recommend to stay away from the dana 70.
Anyway, It came with a nice set all new brakes.
I gutted by stock proportioning valve, They bled just fine...however, no rear brakes ! Fluid is being pushed to the rear, its getting to the calipers, but its simply insufficient to even get the pads to touch the rotors.

I suppose its possible its a bad/worn 35 year old master, So i'll start there. But my hunch is that i now have single piston calipers up from and massive 2 piston calipers in the rear and the stock master simply doesn't has the capacity to push the required fluid to the rear.

Rock auto says the stock master 18m160 has a 30mm / 1.18" bore. After trying a new master, my plan is to find a master cylinder from a chevy with 4 wheel disc that also have a 30mm bore and try that.

Any thoughts??

 

[Recovry4x4]

How did you bleed it? If you bled it by foot, I would guess that you depressed the main seal too far into the bore (which is likely funky) and tore the seal. If you powerbled and didn't over press the brakes, disregard my post.

 

[Tailwheel]

I've spent quite some time researching brake upgrades for my 1985 M1008 to replace the 35+ year old system. In the spring or summer I am going with new DB8 (1-ton) calipers with EBC Yellow Stuff pads and EBC coated rotors up front. In the rear I'm doing the Lugnut 4x4 GM 14-Bolt FF disc brake conversion with DB7 (3/4 ton) calipers, EBC pads and rotors. I'm also doing SS hard lines and ORD SS soft lines. A complete system replacement.

In my research I've found the issue you describe, the master cannot handle the larger volume capacity of the rear calipers. It simply can't move the fluid far enough to actuate the pistons before it runs out of stroke. The solution should be pretty simple and there is an on-the-shelf solution if you want to spend the money. Assuming of course that you don't have any underlying mechanical issues such as blown seals or leaking fittings.

Wilwood Disc Brakes - Master Cylinder Kits: Aluminum Tandem M/C Kit with Bracket and Valve

Wilwood makes a wide assortment of small, lightweight master cylinder kits that are suitable for high performance street and racing applications and they are available in a variety of bore sizes. This page displays Master Cylinder Kits: Aluminum Tandem M/C Kit with Bracket and Valve

www.wilwood.com

^ This Wilwood Tandem Master SHOULD be a direct bolt on with no modifications. You MAY need to lengthen or shorten the pedal rod. The one I linked comes with a manual proportioning valve which I highly recommend. You would likely need to modify your hard lines to fit, but that's not too difficult to do.

You would need the 1 1/8" bore diameter Master Cylinder. The OE equipment is 7/8" if I remember correctly. Going to the 1 1/8" should compensate for the increased capacity of the calipers. The manual proportioning valve will allow you to dial in the brakes perfectly. Should be a nicely balanced system.

 

[camarossguy2]

How did you bleed it? If you bled it by foot, I would guess that you depressed the main seal too far into the bore (which is likely funky) and tore the seal. If you powerbled and didn't over press the brakes, disregard my post.

I dont have the adaptor to attach my motive pressure bleeder to the blazer yet. I used the foot.

 

[Russm]

I ran into the same issue troubleshooting spongy brakes. Wasn't getting enough fluid to rear. Solution was:
- 1980 corvette master cylinder. Same port and bore sizes as stock m1009 (may have to do some line bending for the pickups), but much bigger fluid capacity.
- 2lb residual valve on the rear brake line. That way the pads are almost touching the rotor and don't require much fluid to engage.

Also eliminated the prop valve. Can't get rears to lock up under any conditions and it kept closing so bye bye


Oh ya, big +++ to the motive pressure bleeder. Makes it too easy

Sent from my LM-G710 using Tapatalk

 

[camarossguy2]

I ran into the same issue troubleshooting spongy brakes. Wasn't getting enough fluid to rear. Solution was:
- 1980 corvette master cylinder. Same port and bore sizes as stock m1009 (may have to do some line bending for the pickups), but much bigger fluid capacity.
- 2lb residual valve on the rear brake line. That way the pads are almost touching the rotor and don't require much fluid to engage.

Also eliminated the prop valve. Can't get rears to lock up under any conditions and it kept closing so bye bye


Oh ya, big +++ to the motive pressure bleeder. Makes it too easy

Sent from my LM-G710 using Tapatalk

I've always had great luck with Amazon's "used, like new" warehouse items. Before trying a master for a different application, I found a used like new ACDelco one for a m1009 used like new for$11 lol. So I figured I'd try that first, and it showed up, still new in box
Tried to bench bleed it, installed it, and bleed the brakes. Initially, no change. However I would keep getting a random bubble every once in a while, so I continued to run about 40oz of fluid through it and now it seems well.

I took it for a drive, brakes feel great overall, MUCH better than before.
I check rotor temps after I got back with a laser and the fronts were both 310f, and the treats were 194f
Yet, I couldn't lock any of the tires up on the asphalt.
On my gravel driveway, only the fronts would lock up.

I'm going to buy the $60 adapter to be able to use the motive bleeder, but I think I might be as close as I'm going to get.