i installed the gm 3500 dually wheel cylinders, and ever since i did that the rear wheels break loose when applying any sort of firm brake pressure. What the hell? is there anything i can do?

 

Put the stock ones back on and adjust the shoes every oil change.

 

The MC is designed to push a certain volume of fluid to the wheel cylinders. The volume of fluid will displace the wheel cylinder piston by the same volume. If the piston is smaller, the fluid will push it farther.

I am assuming that the pistons in the GM dually cylinder will be bigger, and therefore displaced less (meaning that the effect on the rear brakes will be smaller).

So I would think the rear brakes would be less likely to lock up?

Is it possible the brakes just need adjusted?

I have also seen odd issues when air is trapped inside the combination valve - maybe try a power bleeder?

 

The upgrade/mod is good if you tow or have a lot of weight in the back. Tools, etc....The upgrade will give you some extra stopping power. And even then, they need to be adjusted regularly because the self adjusters, don't work for shat!

 

That'd be true if it weren't for the combination valve waiting for the pressure on the drum brakes to come up before it starts applying pressure to the discs up front. With larger pistons in the wheel cylinders he gets more physical force for the same fluid pressure -- so the drums are achieving servo action before the proportioning valve is set to divert pressure to the discs.

The fix, if the bigger wheel cylinders are to remain in place, is going to be at the combination valve.

 

+1, it feels like you threw a new set of brakes on the truck every time you adjust them.

 

Thank You, Sheriff, the FSM and the Haynes says to adjust them every 6000 miles miles I think. Just do it every oil change at 3500 miles, and you'll have great brakes and a nice E brake too.

 

did you replace the shoes and springs when you did the cylinders? mine were more aggressive at first as well, probably locked up on dry pavement 3 times or so then they settled down. I figured it was new shoes seating in.

 

When the surface area of a piston increases, the amount of force increases.
Pressure = force/area, so force = pressure x area.

Say you have a 1inch square piston with 1,000 PSI being applied to it. 1,000 x 1in/sq= 1,000 PSI. If you increase the size of the piston to 2 inches/sq, you double the output power. 1,000 x 2in/sq= 2,000 PSI.

 

Shouldn't the anti lock system prevent locking up?