Brake issues
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Trailering
Joined: Aug 2016
Posts: 23
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Brake issues
I have a 1978 F-100 Custom 2wd equipped with non powered disc/drum brakes. When I hit the brakes the pedal holds pressure for a bit then slams to the floor. I have looked all over and no fluid leaks even though the front compartment in the master cylinder is low on fluid sometimes and there is fluid on the outside of the master cylinder. would this be caused by the fact the gasket is no longer sealing cause its almost rock hard? Any help would be appreciated. 
Post Fiend
Joined: Dec 2006
Posts: 9,111
Likes: 3
From: Lexington Mo
It may not be a bad idea to go ahead and get a new lid and gasket for the master cylinder.
If it's been really low before, and you filled it up with brake fluid, you most likely have air in the system. That brake pedal going to the floor, sounds like air in the lines.
Make sure you have enough brake fluid, and bleed all four wheels.
It could also be a sign of a bad master cylinder, and may need to be replaced, or rebuilt.
If it's been really low before, and you filled it up with brake fluid, you most likely have air in the system. That brake pedal going to the floor, sounds like air in the lines.
Make sure you have enough brake fluid, and bleed all four wheels.
It could also be a sign of a bad master cylinder, and may need to be replaced, or rebuilt.
Lead Driver
Joined: Apr 2011
Posts: 6,399
Likes: 40
From: El Dorado, Arkansas
If there are no leaks in the brake system and you hold pressure on the brake pedal for a bit, then, the brake pedal starts to fall towards the floor, the MC is bypassing internally.
Fluid is escaping from the piston chamber and returning into the reservoir through the compensating port in the floor of the MC reservoir.
Time for a new MC (and I would get a NEW one and not a rebuilt one).
Fluid is escaping from the piston chamber and returning into the reservoir through the compensating port in the floor of the MC reservoir.
Time for a new MC (and I would get a NEW one and not a rebuilt one).
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Trailering
Joined: Aug 2016
Posts: 23
Likes: 0
Just replaced the MC cap gasket. The truck stops and holds pressure now other than the air trapped in the lines. Before I change out the fluid in both compartments what is the best method for cleaning out the sludge in the bottom of the reservoir before adding cleaner fluid? I have plenty of fluid to spare.
Lead Driver
Joined: Apr 2011
Posts: 6,399
Likes: 40
From: El Dorado, Arkansas
The MC cap seal really should have nothing to do with the pedal being firm or it falling to the floor. It's only purpose is to keep the brake fluid from sloshing out of the MC and to slow down the rate air/moisture infiltrates into the brake fluid.
Try this; press down on the brake pedal with a moderate amount pressure and just hold it there for about 120 seconds. If the pedal remains firm and at the same height, some miracle has happened to correct the problem.
On the other hand, if after holding steady pressure on the brake pedal for a couple of minutes you feel it start sinking to the floor, a miracle didn't happen and the MC is bypassing internally --assuming there are absolutely no leaks anywhere else in the system.
A bypassing MC will hold pressure .....at first but, if it's bypassing internally, the pedal will begin to sink after steady pressure is applied for a bit, as the fluid and pressure begin to bleed off.
Try this; press down on the brake pedal with a moderate amount pressure and just hold it there for about 120 seconds. If the pedal remains firm and at the same height, some miracle has happened to correct the problem.
On the other hand, if after holding steady pressure on the brake pedal for a couple of minutes you feel it start sinking to the floor, a miracle didn't happen and the MC is bypassing internally --assuming there are absolutely no leaks anywhere else in the system.
A bypassing MC will hold pressure .....at first but, if it's bypassing internally, the pedal will begin to sink after steady pressure is applied for a bit, as the fluid and pressure begin to bleed off.
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Trailering
Joined: Aug 2016
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After pumping the pedal a few times I get good pedal again. I lose it if I don't use the brakes for a few minutes. Due to air getting into the system I am gonna try and bleed the system after I clean out the sludge and dirty brake fluid. Just need to know If I can spray something like brake kleen into my master cylinder to get out the sludge?
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Cargo Master
Joined: Nov 2011
Posts: 3,354
Likes: 48
From: Great American SouthWest
After pumping the pedal a few times I get good pedal again. I lose it if I don't use the brakes for a few minutes. Due to air getting into the system I am gonna try and bleed the system after I clean out the sludge and dirty brake fluid. Just need to know If I can spray something like brake kleen into my master cylinder to get out the sludge?
- When bleeding, you'd just be pushing more sludge into the lines
- A new MC isn't that expensive
Laughing Gas
Joined: Jun 2010
Posts: 1,056
Likes: 10
From: Indiana, PA
After pumping the pedal a few times I get good pedal again. I lose it if I don't use the brakes for a few minutes. Due to air getting into the system I am gonna try and bleed the system after I clean out the sludge and dirty brake fluid. Just need to know If I can spray something like brake kleen into my master cylinder to get out the sludge?
Your earlier issies sound like a bad MC to me though.
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Trailering
Joined: Aug 2016
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Only reason I am trying to not replace the MC is due to the fact I have no Garage to keep the weather off me while I screw around with 30+ year old nuts holding the MC to the firewall, and having to keep the door open to get the rod off the foot pedal which is also probably been on 30+years. If the MC was screwed then why would I have a firm pedal after compressing all the air that is in my system from going low when the seal on the lid was bad and not sealing? Makes no sense that the MC would get a firm pedal after pumping and then the pedal stays afterwards. After 5 pumps I got such good brakes that it almost throws me through the windshield, and it's always consistant 5 pumps. Secondly I don't have $46.00 to replace my MC from my local Advanced Auto Parts Store.
Senior User
Joined: Oct 2016
Posts: 401
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From: Rockville, IN Temporarily
At the least I would flush all lines with new brake fluid. This way you have helped to remove all contaminants as well as any moisture which may be present. Brake fluid should be flushed after several years at minimum or sooner. We flush all lines with each brake job we do. You'd be surprised at how dirty it is on a completely closed system after just a few years.
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Trailering
Joined: Aug 2016
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That I don't mind doing considering I have 3-4 big bottles of brake fluid to do a complete flush. It's just that after getting my brake pedal to respond after changing the lid gasket these people think its still the MC. If it was the MC I'd still have no pedal. The old gasket was so hard and impregnated with rust particles that it wouldn't seal and I'd lose my brake fluid. So technically it wasn't 100% closed.
Cargo Master
Joined: Nov 2011
Posts: 3,354
Likes: 48
From: Great American SouthWest
[QUOTE=Trucklover22;16664651It's just that after getting my brake pedal to respond after changing the lid gasket these people think its still the MC[/QUOTE]
Of course you're free to perform your repairs/maintenance as you think best. As a "new guy" here, just remember there are many years of experience here providing the best advice we can, with a limited understanding of the situation
Personally, I appreciate all the advice I get here, even the advice that doesn't pan out
Of course you're free to perform your repairs/maintenance as you think best. As a "new guy" here, just remember there are many years of experience here providing the best advice we can, with a limited understanding of the situation
Personally, I appreciate all the advice I get here, even the advice that doesn't pan out
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