73 F100 stiff brake pedal

H

hoss73ford

Senior Member
A question I've been wanting to ask for about 5 years. Truck has manual brakes. New master cylinder, front pads & rotors. It stops but I have to bear down quite hard. I'm thinking maybe faulty brake proportioning valve?
 
Was the replacement master cylinder the same part number with the same diameter piston as the old? If wrong and smaller, the brake pedal will be as described.
 
streetdreams said:
Was the replacement master cylinder the same part number with the same diameter piston as the old? If wrong and smaller, the brake pedal will be as described.
Click to expand...

I think quite the opposite is true, the larger piston requires more force to build the same pressure.
 
I think you are mistaken. F= P x A. Larger piston has more surface area, therefore the force you exert is multiplied over a larger area. A power assist master would have a smaller piston because of the assist. Also, 4 of us have responded and no sign of the originator of the issue.
 
streetdreams said:
I think you are mistaken. F= P x A. Larger piston has more surface area, therefore the force you exert is multiplied over a larger area. A power assist master would have a smaller piston because of the assist. Also, 4 of us have responded and no sign of the originator of the issue.
Click to expand...


You are correct about F= P X A, except you've got master cylinders confused with wheel cylinders, and pressure input v.s. force input.
As the piston area of a master cylinder increases, the pressure out will be lower with a given input force. Psi (out)=Force (in) /A.
As the piston area of a wheel cylinder increases, the force out will be higher with a given input pressure. Force (out) =Psi (in) X A.
Power assist is needed with large master cylinder bores to keep pedal ratios and travel within reasonable limits. :)
 
Last edited:
thanks guys for the input. Last owner replaced the master cylinder so no clue if it was a correct match (he might have put in a 67-72 or similar) Motor's coming out for a rebuild & after the engine compartment gets a good bath, may switch to power brakes since the masters & the boosters are readily available
 
You must log in or register to reply here.
Back
Top