Power Brake Booster

[Denis]

It is my understanding that a vacuum brake booster has several petal pumps after the engine dies. Is this true?

If so, I have a problem with my booster. About 5 seconds after the engine is shut off there is no assist. I have replaced the check valve and grommet and inspected the hose and still no better.

Any ideas where to look for a leak or does this mean a new booster unit?

Thanks

[Charles Talbert]

It is my understanding that a vacuum brake booster has several petal pumps after the engine dies. Is this true?

If so, I have a problem with my booster. About 5 seconds after the engine is shut off there is no assist. I have replaced the check valve and grommet and inspected the hose and still no better.

Any ideas where to look for a leak or does this mean a new booster unit?

Thanks

You need to install a vacuum reservoir between the vacuum source, (intake manifold or vacuum pump). We build ours from 5" exhaust pipe tubing, usually around 12" to 18" long. This will reserve enough vacuum for 3-4 good boosted pedal applications after the engine is shut down. If you do not have stored vacuum, you will have no boost when the engine is not running. You will need a check valve on both lines into and out of the reservoir. We most of the time are using a pump since most of our applications are diesel, I use a check valve at the pump inlet and another at the booster outlet or line going from the booster to the reservoir.

[Denis]

Vacuum Storage

Yes, the storage idea is a good one; however if there is a leak some where in the booster this won't work.
I was asking about trouble shooting an internal booster leak to verify I had a leak before paying for a new booster.

[Charles Talbert]

My point is this

Yes, the storage idea is a good one; however if there is a leak some where in the booster this won't work.
I was asking about trouble shooting an internal booster leak to verify I had a leak before paying for a new booster.

Obviously you are not familiar with how a booster works, it is basically dormant until pedal pressure is applied. It takes fluid pressure to activate the booster.

*FACT* You will NOT have boosted brakes with a pedal application when the engine is not running UNLESS YOU HAVE A VACUUM RESERVOIR. This answers your original question as to your "understanding of having several pedal applications" with the engine not running. You will have some brake, but not boosted brakes. If you are using this point and thinking your booster is bad because you don't have a few boosted applications left after engine shut down, it is not a valid test. Both fluid and vacuum diaphrams within the booster will go bad over time. If the booster has been in service for a long time, you could easily have some issues in play and replacing it is not a bad idea, however the usual situation with a booster is this; it simply works or it does not in the majority of cases. If you hear a constant pulling of vacuum through the atmospheric vent when you step on the brake pedal, the diaphram has given up, and has blown, the sound of air being pulled in will be obvious. This is the best tell tail sign that a booster has stopped doing its job. Another obvious point is this; if fluid keeps getting low in the master cylinder reservoir with no sign of external leakage, the fluid diaphram in the booster is blown. Brake fluid is then being pulled into the intake manifold through the vacuum line and burned in the engine. If you have boosted brakes that feel, sound, and work properly with no fluid loss, and are functioning properly in a normal circumstance with the engine running, my instinct is you do not have a booster problem of significance. Here is what you must visualize about boosted applications with the engine OFF. In order for the booster to receive vacuum, obviously it must have a constant source of supply. The source is constant with the engine running, once the engine is shut down, that source is immediately GONE unless you have a reserve supply reservoir within the system. There must be properly oriented check valves installed so the system reserve vacuum can not escape from the reservoir until the brake pedal is applied to activate the system. A proper understanding of how the brake booster actually accomplishes its function will help you see and understand this much better.

When you put pressure on the brake pedal, fluid is delivered to the booster fluid diaphram, this opens the vacuum passage allowing the vacuum source to pull the large piston forward against spring pressure while at the same time atmospheric pressure is drawn in on the back side of the booster piston to equalize pressure within the unit. This large vacuum piston in turn pushes the fluid piston forward forcing brake fluid under pressure to the wheel cylinders to apply the brakes. When the pedal is released, it all goes into reverse, the fluid diaphram closes, the vacuum piston is forced back by spring pressure which forces the air drawn in on the back side out via the atmospheric vent thus releasing hydraulic pressure from the wheel cylinders. Much less pedal pressure is required to activate the booster than it would take to activate the wheel cylinders if a booster were not used, thus the purpose for having the booster in the system.

Bottom line; no vacuum, NO BOOSTER ACTION. Engine not running, there is no vacuum unless there is a reservoir with check valves to hold some in reserve. That should answer that question.

If you wish to have some reserve boosted brake applications after the engine is shut down, you must install a vacuum reservoir. That is the smart thing to do anyway from a safety standpoint. If engine failure occurs unexpectedly, you will have plenty of reserve vacuum to apply brakes in a normal manner and safely get off the roadway before losing boosted brakes.

[712edf]

Very good & informative Charles!

Bucky

[Jarebjr]

"If you have boosted brakes that feel, sound, and work properly with no fluid loss, and are functioning properly in a normal circumstance with the engine running, my instinct is you do not have a booster problem of significance."


Charles,
How does a properly operating booster "sound"? I recently purchased a 1970 W200 with booster brakes and the original owner said he wasnt sure if the booster was working properly. When I apply the brakes...the truck stops well with minimal pedal effort indicating to me that the booster works. I would think much more pedal effort would be needed to stop the truck with just manual brakes. When I step on the pedal...I get a momentary sort of "swish" sound as the pedal is depressed. The sound stops as the pedal reaches travel and the brakes are applied. Is this the "normal" sound of a booster that you wrote about?
Thanks
John Brennan

[Denis]

Vacuum Reserve

Charles;

Great reply. However, doesn't the booster itself act as a vacuum resovoir if the check valve on the hose to the engine is good? This should give at least 1boosted brake attempt after the engine is shut down.

Denis

[Charles Talbert]

"If you have boosted brakes that feel, sound, and work properly with no fluid loss, and are functioning properly in a normal circumstance with the engine running, my instinct is you do not have a booster problem of significance."


Charles,
How does a properly operating booster "sound"? I recently purchased a 1970 W200 with booster brakes and the original owner said he wasnt sure if the booster was working properly. When I apply the brakes...the truck stops well with minimal pedal effort indicating to me that the booster works. I would think much more pedal effort would be needed to stop the truck with just manual brakes. When I step on the pedal...I get a momentary sort of "swish" sound as the pedal is depressed. The sound stops as the pedal reaches travel and the brakes are applied. Is this the "normal" sound of a booster that you wrote about?
Thanks
John Brennan

From what you have said, I'd say all is normal and performing as it should.

[Charles Talbert]

Charles;

Great reply. However, doesn't the booster itself act as a vacuum resovoir if the check valve on the hose to the engine is good? This should give at least 1boosted brake attempt after the engine is shut down.

Denis

Unless you have some type of booster that I'm unfamiliar with, no there is no reserve held in a large enough volume to mean anything. What would be in the hose would simply not be enough to matter much at all. Think back to my booster function explanation, the vacuum piston is pulled forward to apply brakes against spring pressure. It takes more than a dab of vacuum reserve to make this happen.

I'm wondering if you are possibly thinking the large canister portion of the booster assembly is in some way a reservoir. The answer to that is no it is not, that houses the large vacuum piston assembly, there is no vacuum force inside the canister until the fluid diaphram opens the valve to allow the piston to be pulled forward either by a constant vacuum source or by a reserve source, (reservoir). The fact is solid and remains, there will be no vacuum when the engine is not running unless you have a reservoir.

[Doc Dave]

Charles,
I built the vacuum reservoir as you described after a visit to your shop. But I could not remember exactly at the time and did not place a check valve between the reservoir and the booster. I have a check valve up near the engine, and another one just "upstream" from the reservoir.
Will this still work, or do I need to re plumb it to what you said, with the oether check valve between the booster and reservoir?
Thanks

[Charles Talbert]

Charles,
I built the vacuum reservoir as you described after a visit to your shop. But I could not remember exactly at the time and did not place a check valve between the reservoir and the booster. I have a check valve up near the engine, and another one just "upstream" from the reservoir.
Will this still work, or do I need to re plumb it to what you said, with the oether check valve between the booster and reservoir?
Thanks

Hey Doc,
You need a check valve on both sides of the reservoir, I like one at the source near the engine and one near the booster on the other side. This allows storage all the way from the booster, through the reservoir, and back to the source at the engine. The more space between check valves, the more reserve on hand.

[Doc Dave]

Thanks Charles,
Will do.

[PWFFnut]

rebuild parts

I got a parts truck a few years back the frame is totally rusted out and only good for parts. One thing that I saw that I want on My 1956 pw is the brake booster is it something I can rebuild or should I use it as a core & buy one from Vintage pw

[Charles Talbert]

I got a parts truck a few years back the frame is totally rusted out and only good for parts. One thing that I saw that I want on My 1956 pw is the brake booster is it something I can rebuild or should I use it as a core & buy one from Vintage pw

What we do is this, check with your local NAPA, Car Quest, etc. They may can exchange it for you, if there are none like yours in the system, they can take your core unit and send it in for a rebuild. You will then get the same unit you sent in back, all rebuilt to new specs and ready to go. We have never waited longer than 10 days, most often it is faster. We have had very good results using this process.

[Doc Dave]

Charles and all,
I encountered a mysterious problem the other day with my '52 civi power wagon.
A month or two ago I had a problem with the brakes gradually "locking up" after I drove it for awhile (with a few brake applications). I discovered my push rod length adjustment to the master cylinder was not right. I adjusted this to give the brakes more free pedal travel before actuation and it seemed to help. Several short trips around the farm then showed no problems and the brakes worked fine. The free travel of the master cylinder push rod is about 3/4-1 inch now.
The other day I drove it, however, and the same problem occurred toward the end. After sitting still for 10 minutes though , the brakes "let off".
What alerted me to the problem was two things. FIrst the brake lights stayed "on" without pushing on the pedal, and of course I could not push the truck to roll with the brakes on.
Could the rebuilt Midland booster I have plumbed in the system be part of this problem too? Or is it still an issue with the master cylinder in some way?