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'73 Brake Question

ehkrupp

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ehkrupp New Member

ehkrupp

Posted
Posted

I've got a '73 C20 that has an issue with a really soft brake pedal when it's cold. The problem seems most common when it's in the 30-40 degree range.

 

When driving, the pedal will travel basically to the floor with little or no braking action. The brake warning light will also come one. I can get some braking action at the bottom of the pedal travel, but it's minimal. A couple of times, I was able to pump the brake pedal a few times and get the light to reset off and the pedal return to normal, but in the last day or two, no amount of pumping will solve the issue.

 

Once the truck has warmed up considerably (like the first line on the temp gauge above the bottom line), the problem seems to go away. I had driven it slowly (25 mph and below for about 15-20 minutes) today before the brakes went back to normal. It literally seemed like a switch was tossed when the light reset and the brakes went back to normal.

 

I've replaced basically everything from the hard brake lines at all 4 wheels in the last couple of years, the latest being the front wheels in summer of 2009. There were no braking issues until I got to the cold temps in Nov./Dec. Once it got really cold (I'm in PA), the problem didn't seem as severe. The master cylinder has fluid in it and the system bleed just fine when I did the front brakes.

 

I can follow my way through project directions, but diagnosing the issue isn't my strong point. Any thoughts or suggestions would be welcome. Thanks!

gmtech4 Senior Enthusiast

gmtech4

Posted
Posted

pressure bleed the system again and if it acts up again with NO leaks ,replace the master cyl and do not forget to bench bleed the new unit (instructions are enclosed with the new cylinder).

 

temp making a difference makes no sense.

 

don't drive it untill its fixed please.

ehkrupp New Member

ehkrupp

Posted
  • Author
Posted

Would/could a master cyl going bad behave in on again/off again manner? I had no issues with it for a couple months, then a couple days, and then another couple of months with no issues before a day or two ago.

 

One more question - any chance it's related to the vac line on the booster, or does that display some sort of other symptom?

 

Thanks!

gmtech4 Senior Enthusiast

gmtech4

Posted
Posted

m/cyls can behave badly for sure ,very simple hyd system,

 

the vacuum booster stores vacuum that multiplies pedal effort ,thats it .

 

if the m/cyl is old and no leaks ,flip it and BENCH bleed the new one .then pressure bleed the rest of system.

 

vacuum booster can be checked by simply running up engine and waiting awhile and pull out the fitting and if it sucks air in audibly then its fine .

 

with no vac assist ,it takes alot to stop the veh .

 

also foot on pedal slightly, with no vacum in canister,start the truck ,pedal should go down slightly after start up

 

 

 

 

your welcome

  • 4 weeks later...
ehkrupp New Member

ehkrupp

Posted
  • Author
Posted

Just wanted to follow up and say thanks to everyone who posted. I swapped out the master cyclinder and all is well with the truck! The advice was helpful.

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