Bad Check Valve???

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OwdKasd

OwdKasd

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On my 74 with power brakes, As I come to a stop, The Brake pedal goes further and further down. The brakes still work and there are no fluid leaks. The pedal stays real low and then as soon as I turn off the engine the pedal come back up. I'm guessing the check vale is the culprit. Anyone ever deal with these symptoms???:banghead:

Thanks. OK
 
Power Brakes I assume?,....Got a Vacuum pump?. If so, Check your check valve, I believe should have a restriction going one way and easy flow the other,...correct me if I'm wrong,...and check your booster assembly as well,...
 
Thanks. Yep power brakes. It's definitely weird how the pedal comes back up all the way once eng is shut off. I gotta search for the vacuum pump or buy another.
DSC00162.jpg
 
Btw, I put on a new Master cylinder and the fluid had definitely been leaking into the booster. can that fluid be causing a problem or would it have been "sucked" out through the vacuum line???
 
I'm guessing that if fluid did leak into the booster, It might make its way into the vac line,...
 
I'm guessing the fluid is bypassing in the master cylinder. you will never find a leak and the pedal may seem normal at times then other times nearly go hard to the carpet
 
66plyValiant said:
I'm guessing the fluid is bypassing in the master cylinder. you will never find a leak and the pedal may seem normal at times then other times nearly go hard to the carpet
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Okay, I'll try another new MC. Of course I'll troubleshoot the Check valve too. The pedal never feels spongy and never drops quickly. It just keeps going lower and lower with each application.
 
OwdKasd said:
Okay, I'll try another new MC. Of course I'll troubleshoot the Check valve too. The pedal never feels spongy and never drops quickly. It just keeps going lower and lower with each application.
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thats a pretty good sign of a bypassing master cylinder. I think the technical term is bypassing on the primary piston of the MC.

also check the booster when you take the MC off for signs of a fluid leak before you unbolt the lines
 
Yep, The fluid did drop down a bit in the larger reservoir. Since I installed it. So This may be a bad MC that they sold me at Advance auto parts. It still seems like maybe some Vacuum problem because it comes back up instantly when the engine is turned off. That's the part that has me baffled.
 
If the master is bypassing you should be able to duplicate the problem with the engine off, assuming it's a repeatable problem.
 
You should be able to get at least one pump with vacuum assist with the engine recently shut off, if the pedal goes hard as soon as you shut the engine off then either the check valve is bad or the booster has a leaky diaphram. It sounds like the master cyl is bypassing internally along with the booster not holding vacuum.
 
green1 said:
Usually, the fluid will eat the booster diaphram.
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I doubt it, since brake fluid is not a petroleum product like oil or gas it does not eat rubber (this is why you don't put anything but brake fluid in any braking system), however it will remove paint better then any paint striper so don't get any on painted surfaces
 
Another Question: Is the seal between the Master Cyl. and The Booster supposed to be air tight? Or can there be a vacuum leak there? The Master was Definitely bad because the fluid was continuing to drop in the reservoir for the front brakes. Definitely no leaks in the lines/hoses but wet behind the Master. I guess it's hit or miss on the re-manufactured MC's.
 
Well, I installed another M/C and bled it out etc...Just came back from a little cruise around town. Something's still not right with the Booster/Master. It will gradually drop down further and further with every stop. Then when I shut off engine, the Pedal pops back up. It's like something's applying the brakes even when I'm not stepping on the pedal BUT there's no slowing down. In other words it feels like another bad M/C. These are A-1 Cardone's. It's definitely better than before but still not quite right.
 
If you have access to the tools to take apart a booster, you will see what brake fluid can do.
 
green1 said:
If you have access to the tools to take apart a booster, you will see what brake fluid can do.
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Not in my cars since I use silicone fluid, though that starts some people ranting.

It is simple to take a booster apart, if the Midland-Ross type with large band. No risk messing up, since no loose parts inside. I did so in my Newport and found a torn, brittle diaphragm right were the vacuum hose attaches, despite an in-line filter. No replacements available so I cut a new diaphragm out of neoprene sheet. Seems to work fine in a bench test, but may pull loose eventually.
 
I did not take the booster apart yet but found some major problems. The front brakes were totally shot. Pads were metal to metal. Hoses were ready to explode. Probably real bad on the inside and holding pressure keeping the pedal down. Rear brakes were way out of adjustment. So I put on new hoses & pads and adjusted the rears real nice. Pedal feels great now but raining out so test drive will be tomorrow. Also got the New Cooper Cobra 2257014's on. Pictures to follow.
Thanks for all the help.

P.S. I was told how the Previous owner was such a great mechanic which is why I didn't check all this stuff first. Apparently he just neglected all maintenance items on this car. There's also some looseness in the front end but not real bad. I will check and repair the Front End this week.
 
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