power brake booster check valve

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markfh

markfh

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I'm testing my check valve because of a high hard brake pedal on my 1968 Dodge Dart GTS 340. I understand that if I blow through the check valve from the booster side that if air goes through to the hose side that the check valve is bad. That's what's happening with mine and I'll assume that's correct so I'm looking for a new check valve. Anybody know where I can get one that'll work?
 
Don't get excited yet. The job of the check valve is to keep a vacuum reserve in the booster . So long as the engine vacuum is up, the valve goes along for a free ride. If you have a hard pedal with the vacuum up, the valve is not the cause.
So a little diagnostic is in order here.
When do you have a hard pedal?
Do you or did you,ever get boost assist?
What vacuum numbers are you getting from your engine, both at idle and at maximum.
 
You have to blow and suck to check the valve... (pun intended...)
 
i know rock auto has them, but as stated above you may have a bad booster (that rock auto also sells. best prices i found so far.
 
AJ/FormS said:
Don't get excited yet. The job of the check valve is to keep a vacuum reserve in the booster . So long as the engine vacuum is up, the valve goes along for a free ride. If you have a hard pedal with the vacuum up, the valve is not the cause.
So a little diagnostic is in order here.
When do you have a hard pedal?
Do you or did you,ever get boost assist?
What vacuum numbers are you getting from your engine, both at idle and at maximum.
Click to expand...

I have a hard pedal when the engine is running and I don't get any boost assist during braking. The vacuum at the manifold is a steady 15" at idle and 25" or so maximum.
 
Ok , the valve is not the problem. It may or may not be defective, but that's not the source.
So now we need to determine if the booster is bad or if there exists a brake problem, or if the booster pushrod needs adjusting.
Next question, takes us back to post #2; Has the booster ever, in the recent past, provided assist? Or is it a recent transplant? When the pedal gets hard, is it at the top of it's stroke or somewhere halfway down.Depending on the answers to these questions, will lead us to the next logical, test. But I'll skip ahead anyway.
-The brake pedal should not be hard at the top of it's stroke, especially if the car has 4 drums on it. If it gets hard right away, the pedal cannot activate the booster. The pedal has to travel downwards some to activate the booster.
-Let's assume the pedal travels downward to around 50 % of its travel, and that no wheel cylinders are seized, and that the compensating port in the bottom of the master cylinder is open and functioning, so that the brakefluid is returning as would be normal.Now, that's a lot of assuming.
So that takes us to the booster.If you were to pull the booster off the firewall, you would see a linkrod in there, connecting the brake pedal to the booster guts. This rod has to be just the right length, to transmit the pedal motion to the booster valve, at just the right time and amount, to provide the assist.If it is too short you get no boost. If it get's to be too long the boost will be too much, and too early, making smooth stops very difficult.If it's really, really too long, there's a good chance it will brake the internal valve, rendering the booster junk.
-So that take's us back to the questions. If the booster was working recently, and just suddenly quit, well that would rule out the pushrod length.
-But if it's a recent transplant,not so. An adjustment, or several, may awaken it.
-Now lets get back to the assumptions.If the pedal get's hard at the top of it's stroke, lemme know,and I'll get into that.
 
AJ/FormS said:
Ok , the valve is not the problem. It may or may not be defective, but that's not the source.
So now we need to determine if the booster is bad or if there exists a brake problem, or if the booster pushrod needs adjusting.
Next question, takes us back to post #2; Has the booster ever, in the recent past, provided assist? Or is it a recent transplant? When the pedal gets hard, is it at the top of it's stroke or somewhere halfway down.Depending on the answers to these questions, will lead us to the next logical, test. But I'll skip ahead anyway.
-The brake pedal should not be hard at the top of it's stroke, especially if the car has 4 drums on it. If it gets hard right away, the pedal cannot activate the booster. The pedal has to travel downwards some to activate the booster.
-Let's assume the pedal travels downward to around 50 % of its travel, and that no wheel cylinders are seized, and that the compensating port in the bottom of the master cylinder is open and functioning, so that the brakefluid is returning as would be normal.Now, that's a lot of assuming.
So that takes us to the booster.If you were to pull the booster off the firewall, you would see a linkrod in there, connecting the brake pedal to the booster guts. This rod has to be just the right length, to transmit the pedal motion to the booster valve, at just the right time and amount, to provide the assist.If it is too short you get no boost. If it get's to be too long the boost will be too much, and too early, making smooth stops very difficult.If it's really, really too long, there's a good chance it will brake the internal valve, rendering the booster junk.
-So that take's us back to the questions. If the booster was working recently, and just suddenly quit, well that would rule out the pushrod length.
-But if it's a recent transplant,not so. An adjustment, or several, may awaken it.
-Now lets get back to the assumptions.If the pedal get's hard at the top of it's stroke, lemme know,and I'll get into that.
Click to expand...



This car was on jack stands for 25 years. To the best of my knowledge this car is all stock and original including the power brake booster. I can't be 100% sure but the previous owners stated it as so. I am the 3rd owner.

When I took it down and got it running there was a little assist but the brakes didn't work too well due to the condition of the brake fluid in the old lines.

I replaced all the brake lines, wheel cylinders, master cylinder and the front proportioning valve.

I removed and cleaned the booster and reassembled it as well. Everything looked good.

I bench bled the master cylinder three times, twice on the bench and once on the car.

I used DOT 5 fluid and ended up using copper gaskets in order to seal all the lines.

The pedal gets hard maybe half way in it's travel. With the engine running the pedal and the pedal is pumped gets higher and higher but still deflects some.

I tried pumping the brake to bleed off the vacuum and holding it down half way when starting the engine. There was no change in the pedal. It did not get harder or go down at all.

Now as to the new master cylinder and the original link rod. I ordered the MC from Jim's and it's supposed to be the one for this car. I add that the new MC has chambers that are not the same size as they are in the original one I took off.

Is it possible that the link rod doesn't work right with this MC or is this MC not the right one for this car?

Thanks for the help. It is greatly appreciated.
 
I'm a little confused, maybe didn't read every word. The check valve should let you blow from booster towards intake manifold, but not the other way. Your check was thus sounds good, but you need try the other way too. Anyway, as told, at idle you have vaccum so don't require the check valve. That is just to hold vacuum for the future - full throttle or if the engine stalls. You can buy a new generic check valve in the "Help" section at auto parts if it proves bad.

A hard pedal is good, so not sure what you are concerned about. I didn't read that you have test-driven the car and found it hard to stop. True, that when the booster comes on, the brake pedal usually drops a bit, but I think that is because there is a little resilience in the system. Perhaps yours is really good (new hoses, no air bubbles).

I also use DOT 5 (silicone, not the 5.1 glycol rust stuff). People who never used it claims it is "spongy", but your hard pedal suggests not (my experience too). For kicks, read the silly claims that it actually causes more rust because when water drips into your brake reservoir it won't be absorbed. Perhaps those fools let water drip into their power steering reservoirs too, with similar results.
 
Okay, I heard something that needs clarifying;
"The pedal gets hard maybe half way in it's travel. With the engine running the pedal and the pedal is pumped gets higher and higher but still deflects some."
-This could be normal, or not.There are two possibilities here; 1) If the brake shoes are a little loose and/or if there are 4 sets of them, then each pump pushes a little fluid out, which pushes the shoes out a little. Then when you release the pedal, the residual valves slow the return of the fluid. And an immediately following pump again pushes a little fluid out. And so on. Eventually the shoes are out as far as they can go, having reached the drums and put the pressure to them; and the pedal is hard, right at the top of its stroke. So far so good. But now comes the important part. If you now remove your foot to release the pedal, and it returns to it's parked position, then the return springs inside each drum should return all the shoes to their pre-applied condition, and forcing all that fluid back to the M/C. The pedal should now, again be soft. This is very important. It proves the pushrod is NOT too long.
2) If the pushrod is too long, it may not allow the fluid to return to the M/C. In this case the pedal gets hard after the pumping, and may or may not ever return to the former softness, after removing the foot ; it kinda depends on how much too long it is. Just a hair too long, and the fluid may return after some amount of time goes by. If it is more than about 1/16 too long, the compensating port will be blocked, and no fluid can return.
In either case if the pedal remains at the top or less than about 50% travel the pushrod may never reach the vacuum valve to operate it.
The pedal can remain hard overnight and longer.
So let's get that out of the way first. Does the pedal get soft again? Does it return to it's parked position?
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Now; This is how a good booster should work;It assumes the rest of the system is working properly, and the brake shoes are properly adjusted.
With the engine off, Pump the brakes about 5 or 6 times to bleed any trapped vacuum away that may be stored in the booster.( in your case if the vacuum check valve is bad,pumping would not make a difference). Next, put about 30 or 40 pounds of foot pressure on the pedal, and start the car. Right after the car starts a working booster will drop the pedal, under your foot pressure, about an inch and a half, give or take.
If it does that it's good.
If it doesn't do that it MAY be bad. So what to do? Undo the booster retaining nuts and pull the booster away from the firewall, just enough to see the pushrod. Find the adjusting nut on the end, and turn it CW from the driver's seat; 6 flats or 1 turn, to make it longer. Reattach the booster. Retest.Do not mash the brake pedal, as it is possible to brake the internal valve and then, for sure, the booster will be bad. If you feel resistance in the foot pedal, that is different from before, DON'T force it. Ok, start the engine.
Any difference? If the pedal now drops, you are in the ballpark. From here you can adjust the pushrod length to give you just the amount of boost you like, by fine tuning it. If no difference, repeat the adjustment a half a turn longer.Once you get close it does not take much to become too long, So easy does it.
So, good luck.
 
All good info but... realized I have to address the oil leaks that hopefully accounts for the low vacuum so off goes the intake and heads to do all new gaskets. Should have done this months ago. Got it taken down tonight and should get it back together by the weekend. Then hopefully next week I can get back on the brakes.

Looking for a single good original intake bolt with captive washer now to replace one with broken washer.
 
AJ/FormS said:
Mark
It's been 2 weeks. How are you making out?
Click to expand...

Thanks for asking. I got held up by family and business needs so haven't worked on it since last week. I did get the top and front off and cleaned up.

I was going to pull the heads while I was here but compression test showed 170-175 on all cylinders so I decided to leave it as is.

I'll be putting it back together evenings this week hopefully then I can get back to the brakes.

I have to do some soldering on the radiator as well.
 
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