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Brakes rock hard


Jmackin
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1940 Plymouth with a 440 and mid 70s Monte Carlo brakes. The problem I’m having is my engine is pulling 10-12 in of vacuum. With my brake booster getting vacuum from my engine I have easy to press brakes. They just do not do much to stop the car. I put a vacuum pump on the car that was pulling around 20 and will shut off at 23. With that hooked up to the brake booster the brakes are rock hard. Does anybody know what the issue could be?

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Check that one or more of the brake hoses has not collapsed internally.  Are any brakes dragging?  I had a hard pedal on my '67 Riviera and assumed I needed a booster rebuild.  Then one day I pulled into the driveway and noticed that 'hot brake smell' coming from the rear wheels.  Replaced the rear brake hose and voila' -- no more dragging brakes AND functional power assist.  ;)

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What is the pedal ratio? Bore of master cylinder? Booster size? Disk/Drums? Caliper and wheel cylinder bore? You stated that with the booster hooked up, it is easy to press. Just do not have much stopping power. Sounds like pedal ratio or master cylinder bore size to me. Or air in the lines.   https://techtalk.mpbrakes.com/how-to-series/correctly-calculating-pedal-ratio

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I am not impressed with MPB.

I fought a kit I bought from them for years. They were friendly and suggested lots of solutions but couldn't get the brakes right.

Finally went another way that MPB didnt even know about or suggest. Hydro boost.

 

However, I think the OP may be fighting stuck cylinders.

Or pedal ratio as suggested, bore size maybe but may change the pedal travel.

I have the Corvette style Disc/disc MC's in a few different bore sizes that would make a difference (pedal travel included) but still didn't make for brakes that I liked.

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What size booster do you have? What is the bore on the master cylinder? What is the pedal ratio? What are the back brakes? What type of proportioning valve are you using? These are all fundamental to the design and function of the brake system.

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Pedal travel is another one. I have seen pedals that travel a lot, but where the master cylinder rod bolts on. There is very little travel/throw in the rod going into the master cylinder, compared to the travel in the pedal at the base. Because of the shape or design of the pedal. Most early cars end up with a very long pedal dropping down from the dash. And not much room for travel before the pedal contacts the toe boards after insulation and carpet is installed. Boosters on firewalls do not always land in a good spot for setting up pedal mounts under the dash. Pivot points and angles can really make a big difference when setting up your pedals in a vintage car. Shape of the pedal has to kick out towards the bottom, and that can make movement of your foot from the gas pedal more involved then just pivoting on the heal.  Just the joys of changing things, one thing leads to another. Never had one problem with drum brakes and an old master cylinder under the floor.:lol:

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