Brake booster help

[Toni]

Hello.

I´m still confused with my power brakes    The problem is quite hard pedal and poor rear brake power.  It has 4x discs, front stock calipers and rear new but not big calipers.


I already know that i need to modify the proportion valve disc/drum to disc/disc. But which instruction is better?
http://www.classicperform.com/Instructions/PDF/Prop_Valve_Mod_Instructions.pdf
http://www.mpbrakes.com/docs/orig-guides/vl3359%20convert.pdf

And i think that i buy this master cylinder.  What do you think? is this right to my setup? http://www.doctordiff.com/aluminum-master-cylinder-kit.html  (1 1\32)

I did measure the vacuum and there are 14 in-hg at idle.  Its enough right?


How i can recognize that is this single or dual diaphragm booster?

I did test that at least one diaphragm is fine because there is no leak. Or is it even possible that only one is broken? 

I also test this: i pressed the brake pedal, start the engine and the pedal pressed down little bit.

If you can help i´m very grateful

[303 Mopar]

Call Cass at Dr. Diff. Great guy and knows his stuff without trying to sell you things you do not need. - http://www.doctordiff.com/

I know for sure your MC is wrong.  You have a disc/drum MC and need a disc/disc.  Both res should be equal, so you rear discs are not working because you do not have enough pressure with the small res in the front part of the MC.

[Toni]

Call Cass at Dr. Diff. Great guy and knows his stuff without trying to sell you things you do not need. - http://www.doctordiff.com/

I know for sure your MC is wrong.  You have a disc/drum MC and need a disc/disc.  Both res should be equal, so you rear discs are not working because you do not have enough pressure with the small res in the front part of the MC.

Thanks for the advice.  I throw him a question

[John_Kunkel]

I know for sure your MC is wrong.  You have a disc/drum MC and need a disc/disc.  Both res should be equal, so you rear discs are not working because you do not have enough pressure with the small res in the front part of the MC.

The size of the reservoir has nothing to do with the pressure the MC produces, the larger reservoir in the drum/disc MC merely alleviates one from having to top off the reservoir frequently. All you need to do to make a drum/disc MC a disc/disc MC is to remove the RPV from the front outlet (rear system).

[Toni]

I know for sure your MC is wrong.  You have a disc/drum MC and need a disc/disc.  Both res should be equal, so you rear discs are not working because you do not have enough pressure with the small res in the front part of the MC.

The size of the reservoir has nothing to do with the pressure the MC produces, the larger reservoir in the drum/disc MC merely alleviates one from having to top off the reservoir frequently. All you need to do to make a drum/disc MC a disc/disc MC is to remove the RPV from the front outlet (rear system).

Thanks.  Thats good to know.   I was just going to order new master cylinder.  But maybe i check that first. At least i need to buy new seals to it.

DR.Diff told that i need at least 18n hg vacuum.  but i´l try these mods first. if it not helps enough i buy hydrobooster.

[72Charger-SE]

I know for sure your MC is wrong.  You have a disc/drum MC and need a disc/disc.  Both res should be equal, so you rear discs are not working because you do not have enough pressure with the small res in the front part of the MC.

The size of the reservoir has nothing to do with the pressure the MC produces, the larger reservoir in the drum/disc MC merely alleviates one from having to top off the reservoir frequently. All you need to do to make a drum/disc MC a disc/disc MC is to remove the RPV from the front outlet (rear system).

John, I will be needing to do the same for my car.  Can I ask what an RPV is?  (Sorry for my ignorance)...   

[John_Kunkel]

The Residual Pressure Valve retains pressure (around 10 psi) in the system when the brakes are released. Drum brakes have healthy return springs so the residual pressure doesn't keep the brakes applied. However, most disc brakes don't have return springs so residual pressure will cause excessive drag on the brake pads.

In a factory drum/disc MC the RPV is located behind the brass flare in the front outlet port; it can be removed by threading a screw into the hole and prying it out with a claw. 

[Kern Dog]

Call Cass at Dr. Diff. Great guy and knows his stuff without trying to sell you things you do not need. - http://www.doctordiff.com/

I know for sure your MC is wrong.  You have a disc/drum MC and need a disc/disc.  Both res should be equal, so you rear discs are not working because you do not have enough pressure with the small res in the front part of the MC.

Not necessarily true....
I have a 75 Dart disc/drum MC and my 4 wheel disc setup works great. The caliper BORE size needs to be bigger in the front, hence a big reservior for the front. I also run a drum/drum combination valve. This includes ZERO proportioning. The front/rear bias is determined by the different caliper bore sizes front and rear.
Lots of ways to skin a cat here. I also have a 75 Dart booster, non stock for my car of course.