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Discussion Starter · #1 ·
As some of you may know I am struggling with a manual break conversion on another car. I finally have it for the brake pedal feels pretty good but the braking performance is lackluster. It does not stop as good as one I had power brakes on the car. I have driven it three times with the current master cylinder and I've noticed something all three times. After a few miles the brake pedal is considerably harder than when I started the car. I have a ton of underhood heat in this car and I am assuming the firmer pedal is heat related. I had someone tell me that this is a sign of air in the system yet. I am hoping he is correct and that despite the many many times I've bled it I still have air trapped in it somewhere. Does this sound accurate?
 

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Not likely there is air in the lines. With air in the lines you get a mushy pedal. You are getting a hard pedal. It's possable the under hood heat you are talking about is affecting the master cylinder. Is the master cylinder new? Rebuilt? If it is new it's possable the aluminum piston in the master is swelling from the heat after driving and causing the harder pedal. Maybe try some kind of heat sheild for the master or try a different master.
 

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Discussion Starter · #3 ·
I cannot say that the pedal is rock solid I would say it's comfortable as is the one in my daily driver car. It is a new master cylinder actually the 3rd new master cylinder. It never did it when the car had power brakes
 

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the master cylinder for a power unit is a smaller bore. a manual unit has a larger piston. it's simple pressure (your foot) times (X) area(of the master piston) bigger piston, more fluid pushed= bigger reservoir means if you have large OD piston calipers 2" say and trying to push it with a 1" master you will push hard, with a 7/8 master you push less but further.
ASside my '74 w/ drums when drums out of round will heat up and freeze brakes. some fun i'd say

dD
 

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As some of you may know I am struggling with a manual break conversion on another car. I finally have it for the brake pedal feels pretty good but the braking performance is lackluster. It does not stop as good as one I had power brakes on the car. I have driven it three times with the current master cylinder and I've noticed something all three times. After a few miles the brake pedal is considerably harder than when I started the car. I have a ton of underhood heat in this car and I am assuming the firmer pedal is heat related. I had someone tell me that this is a sign of air in the system yet. I am hoping he is correct and that despite the many many times I've bled it I still have air trapped in it somewhere. Does this sound accurate?
Did the break light on inst.panel flash on for a sec.Make sure it is attached to pressure switch.This will let you know if air is in system!I tryed for weeks on Dis break found bleeder was not on top.Bleeder has to point streight up to remove air?
 

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the master cylinder for a power unit is a smaller bore. a manual unit has a larger piston. it's simple pressure (your foot) times (X) area(of the master piston) bigger piston, more fluid pushed= bigger reservoir means if you have large OD piston calipers 2" say and trying to push it with a 1" master you will push hard, with a 7/8 master you push less but further.
ASside my '74 w/ drums when drums out of round will heat up and freeze brakes. some fun i'd say

dD
I think the manual brake master cylinder is a smaller bore diameter and a power brake MC is a larger bore. The manual brake uses a higher pin location to the fulcrum point and the power brake location is lower from the fulcrum point.
 

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A MC for drum brakes has a check valve that keeps some residual line pressure after you release the pedal so it takes less pedal travel to engage the brakes. A disc MC does not do that. If you use a drum master with disc brakes, the residual pressure valve will cause the brakes to drag.
 

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I think Patman and Shane are on to something. What master cylinder are you using? Also, did you convert the car from a power brake system to a manual system? You said manual brake conversion, but what did you convert? 1st gen, 2nd Gen,3rd gen? Disc brakes or drum? Add some detail and I'm sure folks here can tell you what the issue is.
 

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Discussion Starter · #13 ·
I converted the car to manual brakes from power. The car setup is the same except for this. Aluminum master cylinder before and aluminum master cylinder now (15/16") The pushrod has been mover to the upper hole on the brake pedal. I bought the kit from manualbrakes.com

What I don't understand is why after a couple miles of driving the pedal feel is considerably harder. I found an article where someone had this same issue and it turned out to be the pushrod was adjusted to tight, I need to see if this is my issue. Whatever is causing the issue I'm pretty sure it's due to heat but not sure what the heat is affecting
 

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Discussion Starter · #15 ·
It is a disc/ drum car still using the factory combination/ proportioning valve.

I verified I have plenty of slack in the brake pedal pushrod. I think this weekend I'll take it for a ride and when it happens I want to jack the front and rear up to see if any of the brakes are dragging. It doesn't pull so if that is what's happening it must be both sides.
 

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As some of you may know I am struggling with a manual break conversion on another car. I finally have it for the brake pedal feels pretty good but the braking performance is lackluster. It does not stop as good as one I had power brakes on the car. I have driven it three times with the current master cylinder and I've noticed something all three times. After a few miles the brake pedal is considerably harder than when I started the car. I have a ton of underhood heat in this car and I am assuming the firmer pedal is heat related. I had someone tell me that this is a sign of air in the system yet. I am hoping he is correct and that despite the many many times I've bled it I still have air trapped in it somewhere. Does this sound accurate?
Harder pedal and decreased brake performance indicates fluid restriction..
provided other brake components are in good condition.
That said, make sure mcyl bore dia isn't larger that exact vehicle spec. That also creates a harder to apply amt of pressure to the wheel units. You need about 1200 psi as measured at each bleeder by small hi pressure gage . One of those can save lotsa time diagnosing trickey hydraulic problems for <$100.
M/cyl w/smaller bore than stock (ex: replace oe 1" with 15/16" can be used in place of booster as it produces more pressure with slightly longer pedal travel @ same pedal pressure)
 

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Discussion Starter · #18 ·
I recently purchased a brake pressure gauge kit to help troubleshoot this. Unfortunately the bleeder adapters are very poorly machined so I had to have them reworked. I bought a SSBC kit and contacted them about the fittings that would not seal. They asked for pictures which I sent. All I got from them was a "thanks".


I'm in the process of doing more shrouding of the brake lines and combo valve from heat then I'll try it again. Hopefully this weekend I can measure pressure. Thanks for the help I appreciate it !
 
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