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| | | | Joined: Jul 2000 Posts: 2,191 Insomniac | | Insomniac Joined: Jul 2000 Posts: 2,191 | We were planning to go for a drive in our 1954 1/2 ton today. I started the truck and moved it out of the garage. Tried to stop so we could close the garage door but the brake pedal goes to floor like there's nothing there. Fortunately, the driveway is level so it just rolled to a stop. Pumping brakes did not bring pedal up.
* Truck has power brakes with booster and dual master cyl under the floor.
Checked pedal linkage. Pin was OK and secure. Link and return spring were Ok. Input rod on booster moving in/out as expected as pedal pushed and released.
Checked brake fluid level. Both reservoirs were full. I expected this as I didn't see any signs of leaks.
With engine running, brake pedal goes to the floor easily. With engine off, brake pedal goes to floor with a bit more effort. If the booster was gone, the brakes should still work but with more effort.
My conclusion is that master cyl has to be replaced. Reasonable conclusion or did I miss something?
Gord 🇨🇦
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1954 1/2 ton 235 4 speed
| | | | | Joined: Feb 2004 Posts: 29,773 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,773 | Truck has power brakes with booster and dual master cyl under the floor.
My conclusion is that master cyl has to be replaced. Reasonable conclusion or did I miss something? Do you have residual pressure valves in the ports of the master cylinder? If you're running discs up front, they need a 2# valve, and drum brakes need a 10# residual pressure. If the master cylinder is below the level of the brakes, the residual valves keep enough pressure in the cylinders that air can't creep in and put a bubble in the lines. Even a tiny air bubble will cause the symptoms you're having. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!
| | | | | Joined: Jul 2000 Posts: 2,191 Insomniac | | Insomniac Joined: Jul 2000 Posts: 2,191 | I have Camaro master cyl, combination valve*, front discs and rear drums. The master cyl and booster are tucked up as high as they can go under the floor of the cab so they are higher than the brakes.
The brakes have worked really well for about 1000 miles at this point. Day before yesterday, they were good. Yesterday, they were non-existent.
*combination valve has proportioning valve, metering valve, brake warning light switch built in.
Gord 🇨🇦
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1954 1/2 ton 235 4 speed
| | | | | Joined: Feb 2004 Posts: 29,773 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,773 | Try bleeding the brakes before you start throwing parts at the problem. If it becomes a chronic problem, it's probably time for a new MC, as the failure rates of those third world parts are almost as bad as condensers. Be sure to bench bleed a new one before installing it.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!
| | | | | Joined: Jul 2000 Posts: 2,191 Insomniac | | Insomniac Joined: Jul 2000 Posts: 2,191 | Good idea! Thanks!
Last edited by Gord&Fran; 08/29/2021 7:21 PM.
Gord 🇨🇦
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1954 1/2 ton 235 4 speed
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