power disc brake conversion

[jcolville89]

I have a 69 roadrunner I converted to disc brake I put spindle of 1974 duster with all new calipers and hose I bought a booster and master cyclender kit Im not sure if I have enough vacuum plan on trying a can. I feel the brake booster at Low speed grab but it's not like a new car where I can push down its just hard thinking maybe booster is bad possibly any ideas on something it could be besides that kind of Need help brainstorming would air in brakes cause this I always heard they be spongy not hard. Brake pedal feels like factory drum breaks non power if that help

 

[5.7 hemi]

Did you change the prop valve for a disc drum combo?

 

[jcolville89]

That is the only thing I haven't change but looks to be the same as a disc one

 

[mopar 3 B]

Have you checked manifold vacuum, need at least 10 hg at idle. Master cylinder push rod adjusted to correct length? Check valve in booster connected directly to intake manifold? Master cylinder bleed before attaching brake lines? Brakes bleed properly? Are all components compatible? Vacuum can will not benefit braking only vacuum operated accessories.

 

[Propwash]

That is the only thing I haven't change but looks to be the same as a disc one

It's not. A drum/drum car has a distribution block, that basically just distributes the brake fluid to different circuits. A disc/drum car would have a proportioning valve, which also routes the brake fluid to different circuits, but it also sets a predetermined amount of braking pressure to the drums vs. disc's. It's a must for a disc/drum combo. You could buy an adjustable proportioning valve (sold by folks like SSBC, Wilwood, Summit ect..ect..) and plumb it in the line to your rear brakes, or track down a factory style one. Either way, you need one.

Also, what Booster & Master did you buy? A disc brake booster is different than all drum. Typically dual diaphragm (disc) vs. single (drum). Need the right one of those as well. As far as the master, you need a disc/drum master. It will have a residual valve built into the rear brake port of it and none on the disc side. It's needed as as well.

Air in the brakes will cause spongy, weak or no brakes. Bad booster/low vacuum will cause very hard brakes. So will wrong master bore size for the system. I'd make sure you have the appropriate hardware in place, see how it acts and then go from there. Good luck.

 

[jcolville89]

Think imma try a proportioning valve and see how they go do the all bolt up the same one line in and 4 out to the wheels like mine