Mod the MC to get more rear disc action?
 

I bought this

https://www.burkhartchassis.com/prod...aster-cylinder

which is the Strange Engineering MC.

I've got the Wilwood prop valve all the way open and I'm not getting any more action out of the rear PBR calipers. I've put the front reservoir and the rear to the back with no affect.

Super hard pedal (this is manual bakes) and I can lock the fronts but not the rears.

Could I mod the MC to put more flow or pressure to the rears then control it with the prop valve?

Or am I flirtin' with disaster?

Thx

Re: Mod the MC to get more rear disc action?
 

No, front and rear pressure are same. Only the prop valve lowers the rear pressure.

Re: Mod the MC to get more rear disc action?
 

Yeah that makes sense thanks.

I talked to Strange, they suggested I could get 200 lb more pressure by lowering the hole in the pedal from a 6:1 ratio to a 7:1 ratio.

They said a larger bore MC would prolly just make the pedal spongy.

Will try the new hole.

Re: Mod the MC to get more rear disc action?
 

For more pressure you want a smaller bore master cylinder. Upping your leverage should help , but if you are already locking your fronts you don't need more pressure,

Re: Mod the MC to get more rear disc action?
 

So basically you've got '92 brakes in the back. What do you have in the front? And what type tires and sizes front and rear?

I don't think you're going to have much luck with manual brakes in the stock pivot location.

Re: Mod the MC to get more rear disc action?
 

And what bore diameter is the new master cylinder?

Re: Mod the MC to get more rear disc action?
 

Are you having trouble stopping the car at the end of a 100+ MPH run?

Are you having trouble holding the car at the line when power braking?

The 1992 rear calipers come with a 40mm (1.57”) single pistons. This is actually quite small, but is a normal size for a rear caliper in production cars.

If you have a lot of tire in the back (wide, tall, and sticky), then that small caliper is going to have a hard time locking up something with a lot of grip, especially with a larger bore master cylinder that reduces line pressure and clamping forces at the caliper. A large, wide, sticky tire works both ways by providing traction at the start line when full throttle is applied and providing grip to stop the car. It is just much harder to lock up a sticky tire. The fronts are easy to lock up because the tire is most likely skinny and you have a calipers on the front with a 2.5” diameter pistons which has a piston area that is 60% larger than the rear calipers.

With your current brake system, going with a smaller bore master cylinder and choosing a rear pad that has a lot of initial bite will help, but it still may still my not guarantee it will do what you need. Smaller bore master cylinder will have a longer pedal stroke, but will be easier on the driver to stop the car. With your current setup, I would usually suggest a 7/8” bore master cylinder.

A 6 to 1 pedal ratio would put the pin location two inches down from the pedal’s pivot point on a stock brake pedal. For the stock brake pedal, this gives 7/8” to 1.0” of master cylinder piston stroke. For good push rod geometry, it is best to have the master cylinder mounted to the firewall using the upper two holes that the vacuum booster bolted too to take advantage of the 6 to 1 pedal ratio.

If you move the pin location up on the pedal to increase the pedal stroke, you will have to move the master cylinder up on the firewall to keep all the geometry “in line”. By moving just the pin location up will increase your pedal stoke, but is also decreases to master cylinder piston travel which reduces the amount of fluid volume to the calipers.

Please let me know if you have any additional questions.

Re: Mod the MC to get more rear disc action?
 

Thanks for the responses.

I have 255-50-16's all the way around. Stock 89 PBR single piston aluminum calipers in the rear. I have bigbrakeupgrade dual piston LS1 calipers up front.

The pedal is very hard but the braking action is stable repeatedly from 100mph. I can lock the fronts if I push really hard. I have not tried power braking but I think it would hold.

My goal is see 4 black marks on the road and not just two from the fronts. Is that unrealistic?

Thanks for the feedback!