FireDemonSiC

Question for you guys. Was going through the vacuum lines on my car while piecing the intake back together after a teardown.

Decided to replace the 3/16 line going to the vac reservoir as it looked badly cracked and degraded. As I was separating it from the Tee fitting, I noticed that the nipples on the tee were cracked apart, so I went to the parts store in search of a replacement. Turns out, dorman makes this same part:

http://www.ebay.com/itm/like/191494260405?lpid=82&chn=ps&ul_noapp=true
My question is this however. What exactly is the function of having a checkvalve there? The way the configuration goes is that the check valve has a single 1/4 nipple and 2 3/16 nipples. The 1/4 goes to the plenum/manifold and the 3/16 go to the vac reservoir and the HVAC blend door. The check valve is setup so that it only allows vacuum to be pulled on the engine side, but any pressure from the 1/4 hose will be blocked from getting to the 3/16 hoses.

The reason I am asking is because the doorman part I bought seems to allow a very small restricted amount of airflow to the 3/16 side while the factory valve I removed blocked 100% of airflow.

The only purpose I can think is to protect the vacuum accessories in the event of an intake backfire. Or, is there something more to it?

AlkyIROC

Under acceleration, there is little to no manifold vaccum. If manifold vaccum drops for an extended period of time, devices that use this vaccum will stop working or in things like blend door actuators, may move to a different position. Older cars especially from the 60's like a Caddy has a lot of vaccum line in it. Taking off from a light and the windshield wipers would stop working because they were vaccum controlled.

Putting a check valve in the system, prevents sudden low manifold vaccum from decreasing to all the components in the system.

FireDemonSiC

Good way of putting it. Since a sudden low manifold vacuum would effectively pressurize the intake in comparison to the vacuum reservoir, this makes sense.

Here is my main question though, I suppose.

When you shut the car off, it is supposed to maintain a small reserve of vacuum for a short period of time. This is why you can get in the car days later and still depress the brake pedal with vac assist. Also on cams with a large amount of overlap, it can hold a small amount of vac reserve for low speed driving (Actually found myself in this situation when I first ran the new motor, as incorrectly giving my short travel lifters 1/2 turn of preload severely reduced manifold vacuum).

Is the check valve to retain that vacuum in the vac reservoir in the reservoir itself, or is it that valve I displayed? I am concerned since the one I bought seems to allow a very small, restricted amount of airflow to the accessories side.

sofakingdom

iTrader: (1)

^^^ This ^^^

Virtually ALL cars used to run the wipers (those that even had that new-fangled contraption) off of vacuum. Car electrical systems were simply too unreliable back in the 50s, vacuum was far more dependable.

Brakes have their own check valve, they aren't involved with the one for cabin accessories.

My late little bro once had a car that somebody had "cleaned up the engine bay" real good. Every time he got on the gas after shifting gears (4-spd) he heard a strange little eeeeeking noise coming out from under the dash every time he went through the gears. Turned out it was the defrost/dash vacuum motor restoring itself and dropping back out as the clutch and throttle were cycled. Re-installing the check valve and reservoir that the yutz had "improved" away, cleared it up.

One other system that depends on the check valve, is the cruise control. It's not much fun when the cruise tries to speed the car up, but its act of opening the throttle of course makes the vacuum drop below the threshold of keeping it engaged, so it drops completely out whenever it tries to accelerate too much. Another real head-scratcher.

The valve in your pic is the one for cabin accessories. The brake one is MUCH larger and is in the vac line to the booster.