Bad "Non-Return Valve, Brake Booster" ?
 

I have a situation where I have power brake boost right after I start the car and for 2 or 3 brake applications. Then the boost goes away for a few applications, then returns and is fine all day. This problem seems to have come about this winter.

I haven’t done any vacuum pump testing yet, but I wanted to first ask if a sticking “Non-Return Valve, Brake Booster” can cause these symptoms. Because I have brake vacuum at cold start up I think that means it is working fine at holding a vacuum, but can it “stick” and not allow fresh vacuum until it releases for some reason?

I'm referencing part #140 below.

Thanks.




http://i31.photobucket.com/albums/c3...ineRouting.jpg

Answer
 

Generally this is a symptom of a vacuum pump issue.

Then is seems the first thing I need to test is vacuum pump suction when the engine is cold.

Correct.. :)

Minimum is 13 inches mercury.

Good is 26 - 30 inches mercury.

FYI
 

The vacuum pump was superseded four times.
Need VIN# for correct part..

Here it is:

VIN WDBEB28E1PB901581
Chassis 1241281B901581
Engine 602962 12 016644

you could test the non-return valve by cracking open the flare nut on the booster end of the plastic booster-vacuum pump hose. this is when the car is stopped, but you had power brakes before you stopped. there will a decent bit of vacuum left in this line if the non-return valve is good, so when you crack the nut open it will hiss (sucking air).

I should also mention that I can hear my vacuum pump. Not outside the car but only at idle when sitting in the car. It doesn’t sound bad, as if it ready to self destruct or anything like that. It makes a subtle “chu-chu-chu” noise that goes away if not idling. I can’t say I noticed it before but I am wondering now what a new pump would sound like….or can you not hear new pumps at all?

Answer
 

Vacuum pump
MB# 000 230 31 65
Fastlane: Vacuum pump
http://catalog.peachparts.com/item.wws?sku=0002303165

Answer
 

You can not hear a good vacuum pump...

Took some measurements
 

Last night when I got home I took some vacuum measurements…
Hot Engine (all measurements - in Hg)
23 @ Vacuum pump
23 @ Non-Return valve

This morning I took measurements after starting the cold engine…
Cold Engine
23 @ Vacuum pump
23 @ Non-Return valve

Some other observations:

-When the vacuum gauge was attached to the shut off valve port on the Non-Return valve, and with the engine running, pumping the brake pedal would make the vacuum eventually rise to 10. After ceasing pumping the vacuum would drop back down to 23 again. Is this the correct vacuum pump capacity when at idle?

- Using a mechanics stethoscope on the vacuum pump, and with the engine running, I cannot hear any abnormal sounds coming from the vacuum pump.

This morning when I disconnected my shut off valve port / hose connection, I could hear air sucking into the port. This seems to indicate the No-Return Valve is doing its job and that the brake booster diaphragm is not leaking.

But, driving down my street again this morning (after doing the pumping thing explained above), I lost power assist to the brakes for a few applications. It returned as it usually does.

Conclusions so far…
I am thinking my vacuum pump is OK unless one of you guys says otherwise. Now I need to test my brake booster. How do I do that?

Any other ideas?

Thanks for all your comments so far.

Answer
 

Brake:
http://www.peachparts.com/shopforum/diy-links-parts-category/142411-brake.html

Lot's of good reading in the "Brake" link you posted.

After reviewing the threads, it was clear it needed to be verified that the master cylinder was not leaking fluid into the booster.

1) The fluid on reservoir is not dropping. It has not moved appreciably since the brake lines and pads were replaced this summer.

2) To verify #1, the vacuum line was removed from the booster, a thin tube was inserted down the port to point near the bottom and a vacuum drawn on it the tube with a MityVac. No fluid was drawn out.

3) The Non-Return valve does hold a vacuum in the booster. But it should let air flow past it towards the vacuum pump it seems. Blew on it but could not get any air past it. Tried again and air easily went past the valve. Tried again and nothing. Could the Non-Return valve be the problem?

Next step is to build a tee fitting to place between the vacuum line and the booster to verify the condition of the Non-Return Valve.

Has anyone built one and would share the fittings needed to do this?

Thanks!

Any luck???? What did you end up doing?????