I need some input. My 55 merc still has the 4 drums. The manual single line master had some pedal movement but got very firm with good braking after just a little pedal travel.

I just installed a power booster and dual master. I bled the brakes and now the pedal is rock hard with just slight movement, about an inch or two to full lockup. Pump the brakes and started while holding pedal and it sunk slightly, so i think the boster is working OK. I adjusted the booster rod so the master was not preloaded. I even spaced the master from the booster and got pedal travel until master engaged, then rock hard. Therefore i think it is in the lines or drums.

Any ideas on the problem? And what tactic should i take for troubleshooting?

Thanks

Just throwing this out , when I shut my car off and pump the brake pedal it gets rock hard as the residual vacuum is used up. Maybe your vacuum source is the problem or the check valve is bad .

Let me get this straight, you bled the brakes and the pedal was STILL rock hard? Bled from where? When the pedal is hard and you apply your brakes are all four wheels locked up? (Like tires off the ground can't turn any of them locked up?). Like you implied, there seems to be some blockage somewhere, certainly not enough brake fluid being pumped to expand all four wheel cylinders if the pedal gets hard that high. Do you have a proportioning valve? Maybe you're only getting flow to the front wheels. I believe there's a pin on the proportioning valve that you can tap back in if the valve is stuck.

" If you always do what you always done, you'll always get what you always got" - Henry Ford

You said that you installed a new dual master cylinder as well. Did you bench bleed the master prior to installation?
Rono

  Ron Lane,  Meridian, ID

With what you have described I cant see what the problem is>.??  You do mean locking up the brakes?  1-2 " travel seems ok ...?



"Came too close to dying to stop living now!"

I got some clear weather to get the car out and drive it around the parking lot.  The brakes work fine and I can lock up all four easily.  So the booster is working.  The pedal is high and only has an inch or two travel to be rock solid.  I guess I was expecting a longer/softer pedal travel.  The car has always stopped really well for 4 drums, it just took a lot of leg.  Now it is almost effortless.

I'm curious why you just didn't add a vacuum booster on the fender as the dealers did when a customer wanted power brakes. I have the same system you have on my 56 and feel it is very adequate other than the pedal needing extra effort since the brakes are manual. When my father bought his new 55 Sunliner that's what he did and when I drove it in the 60's as I remember it was a pleasure to drive. I've considered discs up front and may not use a dual master cylinder just doing what the 65 Mustang did with discs up front.As I remember my brothers new 65 Fastback had discs with no power and a single master cylinder...

56 Vic, B'Ville 200 MPH Club Member, So Cal.

I take it its the Corvette style M/C ? Perhaps its the wrong bore size.. Normally 1" but they do make others..?
I finally worked out what was wrong with my brakes after installing the dual M/C and booster. I had one wheel cyl seized , old brake lines and despite great looking brake linings the fronts were standard size when the drums was over size.. Doh!
I made up all new lines, honed all the wheel cyls, and got the correct sized linings and bingo! Best brakes ever... Unlike yours however my pedal is quite progressive.  



"Came too close to dying to stop living now!"

You have added power brakes to a pedal set up for non power brakes , your fulcrum on your pedal is set up for manual brakes . see if there is an alternate hole in the pedal to attach the brake push rod to. It all a matter of geometry your current set up requires less effort than the manual brakes did.

Yes there is a 1.125" MC that most PB supplier's send in their kits. I have power disc on my car. I had to change the MC to a 1" bore and change rear wheel cylinders to 15/16"  from original 7/8"