70 Disc Brake Conversion Problem

[BlazerBrad70]

So I converted my 70 4x4 blazer to Disc/Drum set up and the brakes are acting like there is no boost assist. I've got a new V8 350 5.7, all new brakes front and back, new master and brake boost, new valve... don't know what else to check... help please...

[jayman68]

I would suggest first verify that you have good vacuum to actuate the booster. Some cam profiles do not create enough vac to power the booster. you may need an electric pump, or a hydraulic system. Good luck.

[BlazerBrad70]

i got plenty of vacuum, i went from a power drum/drum set up to a power disc drum set up with everything new...

[MB_70]

I had this problem ..... it was a combination of air in the lines and the wrong master cylinder. One of my symptoms was that we couldnt get any pressure when bleeding the system with a helper. I never had a reason to suspect this, but it was a $40 part that caused a $500 problem. If you bought the mastercylinder separate from the booster, it could be worthwhile to double check compatability through the vendors.

If that checks out, I would suggest bench bleeding the master cylinder, then gravity bleeding your brake system (if not done already). After gravity bleeding you will need to pressure bleed your system all over again with a helper.

You should also double check all of your lines to make sure they are not crushed, crimped, folded, and uniform in size. Also check the rubber lines to make sure they are not leaking.

Not trying to offend or belittle any skill sets or experience. Your post wasnt too detailed, and one thing we all have over looked (self included) is the fact that these are 40 yr old trucks that have usually been off road at one point in their lives. Mechanical skills vary person to person, as does integrity.

Its something small that is frustrating you right now. Best of luck, and let us know what it was ... might save someone else an afternoon of misery.

[BlazerBrad70]

so changed the Booster and now she brakes like a dream. Thank you guys for the input, i will take any advise anyone has to give, you never know what the guy next to you knows...