Over-sensitive brakes — what causes them?
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4wd Low
Joined: Jan 2019
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Over-sensitive brakes — what causes them?
Hello all,
I recently acquired a 68 F-250, power-assisted brakes, front disk, rear drums. The brakes are overly sensitive, I.e., even very slight braking causes the wheels to lock up. What causes this? I’ve read everything from a badly adjusted booster pushrod to a pedal that’s too long to a bad booster. What’s the best troubleshooting process? BTW, I’m 3000 away from the truck, haven’t driven it yet, and am getting the symptoms 2nd-handed from my son. I’d like to give him a checklist for systematic troubleshooting...
Many thanks,
Bob
I recently acquired a 68 F-250, power-assisted brakes, front disk, rear drums. The brakes are overly sensitive, I.e., even very slight braking causes the wheels to lock up. What causes this? I’ve read everything from a badly adjusted booster pushrod to a pedal that’s too long to a bad booster. What’s the best troubleshooting process? BTW, I’m 3000 away from the truck, haven’t driven it yet, and am getting the symptoms 2nd-handed from my son. I’d like to give him a checklist for systematic troubleshooting...
Many thanks,
Bob
Fleet Owner
Joined: Aug 2010
Posts: 21,255
Likes: 1,657
From: Reynoldsburg, Ohio
Having once driven a late 60's "power assist" front disc brake vehicle those systems were already very sensitive to any brake pedal action---in those days the factory's had not developed the systems to work smoothly in conjunction with one another.
FWIW my 2000 E-250 has front disc & rear drum brakes and they work well together---amazing how 32 years development smooths things out.
FWIW my 2000 E-250 has front disc & rear drum brakes and they work well together---amazing how 32 years development smooths things out.
Lead Driver
Joined: Sep 2003
Posts: 7,018
Likes: 210
From: SoCal
Years ago, when I was changing the master cylinder on my 69 Mustang, I accidentally dropped the little rubber pad that sits on the end of the rod that goes into the little cup in the middle of booster's diaphragm. I dropped it into the booster's vacuum chamber, so it was impossible to retrieve without removing the booster. So I decided to adjust the rod to try to compensate for that small distance by cranking out the threaded stud on the end. After assembly, I found the brakes to be hyper sensitive. I later pulled the booster and shook out the little rubber pad, put it back onto the push rod, and carefully put it back into the booster as I re-installed the MC. The brakes behaved much better after that.
So if the problem is really bad, you might want to pull the MC and look inside the booster where the push rod goes into the cup on the diaphragm and see if you still have that little rubber pad, or is it pushing metal on metal.
So if the problem is really bad, you might want to pull the MC and look inside the booster where the push rod goes into the cup on the diaphragm and see if you still have that little rubber pad, or is it pushing metal on metal.
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 219
From: W (BY GOD) V
You need a DIAGNOSTIC SEQUENCE - https://www.mpbrakes.com/Support/Tech-Support.asp
They have a download on that page.
They have a download on that page.
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devino246
1973 - 1979 F-100 & Larger F-Series Trucks
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Nov 18, 2011 07:48 PM
