Thoughts on hard brake due to vacuum venting - PeachParts Mercedes-Benz Forum

fezcabdriver · #1 01-27-2017, 08:49 PM

So I have a 4runner om617 transplant. The brake gets seriously hard when i use in succession. I'm wondering if I'm not able to generate enough vacuum due to how my IP shutoff is setup. The only thing using vacuum off of the main line is the booster and the IP shutoff so this brake booster vac line is not the OEM one. It is a vacuum line that come from pump --> check valve(i think)--> t junction to booster
The vac line from this junction goes to a 3/2way solenoid. When ignition is turned, 12v is sent to solenoid where it closes circuit to IP but passes to vent tube. I'm not sure why the PO did this. I understand the vent should happen on the IP side so that residual vacuum can bleed off and the car can be restarted. Anyway, if you read this far, thanks. What I'm wondering if this vent is enough to cause my brake issue. Recall that the check valve on this make-do vacuum booster cable is at the bottom near the pump and not past the junction as it is on standard w123 setups. Could the placement of the check valve in combination with that vent be my problem?
I haven't tested it as I'm not near the truck, just wondering before I tackle. THanks for reading.

SD Blue · #2 01-28-2017, 11:11 AM

I'm not clear as to how the switched set-up is working. From my understanding of the diagram, the vent is open to the pump constantly. Without it, I would think that the vacuum would be working on a very small volume.

Here is a diagram for the W126 (post #5) the to Tank? comment is the vent line.

fezcabdriver · #3 01-28-2017, 12:30 PM

Yes, that is what I'm trying (and failing) to communicate. It looks like the when the solenoid is closed (car running), the supply line is constantly venting to atmosphere.
Does that sound wrong? Overworking my pump?

When I get a chance, I will further investigate, but does putting a check valve from t junction to solenoid seem like a feasible option so I don't lose any vacuum at booster. Recall, it looks like the check valve on this setup is immediately after the vac pump and before that junction.

Thanks

SD Blue · #4 01-28-2017, 12:59 PM

What my thinking is, the vent has to be constant since the vacuum source is constant. This regulates it at a certain value.

fezcabdriver · #5 01-28-2017, 02:21 PM

So if i'm understanding you correctly the vacuum system should vent during operation?
Regarding my setup, I just plugged the vent and will go for a drive to see if I lose brake booster.

I guess my remaining questions are..
1. Is the vaccuum system supposed to vent during operation?
2. Did the PO put that check valve closer to pump because maybe the valve connected directly to the pump was broken and this was his work around so that he didn't have to buy a $80 part??

I'm going to block off the t junction just so that it works only on the brake booster to see if I have same/better results.

What should my vacuum measure directly from pump? or elsewhere? 22hg?

barry12345 · #6 01-28-2017, 03:11 PM

I may be well off base but you may need a vacuum reservoir. As mentioned I am not applying much thought here.

SD Blue · #7 01-28-2017, 03:30 PM

Most vacuum systems, not automotive, have sensors or regulators that shut the pump down when a certain amount is reached.

I'm thinking that a certain amount of venting is needed in lieu of regulators or cut-off set up.

dieselbenz1 · #8 01-28-2017, 06:48 PM

Disconnect the vaccum to the solenoid, plug it and test your brakes. I also think the solenoid is plumbed in incorrectly.

fezcabdriver · #9 01-30-2017, 01:11 AM

Thanks dieselbenz1. For the time being, I've eliminated the shut off and capped off at the junction. Went for a drive, stomp on brakes 3 -5 times and then it still gets hard. Now that I've ruled out shutoff and can pretty much recreate the "problem", can anyone comment on whether or not vacuum should generate almost instantaneously or is this normal operation to mash 3-5 times consecutively and have to wait like 5 seconds for build up of vacuum?
OR
does it look like my pump is failing? I'm disconnecting the booster line tomorrow and testing my pump directly with a mityvac? I'm assuming I should see a constant 20Hg minimum. Also, that check valve should be inspected.

Thoughts??

dieselbenz1 · #10 01-30-2017, 03:58 AM

Test your pump you should get around 20 inches vacuum. A bad check valve i dont think is your problem. You should not have to wait for vacuum to build up.

mach4 · #11 01-30-2017, 09:48 AM

Mashing the brake 3-5 times in rapid succession will deplete the vacuum giving a hard pedal, and regeneration will take a few seconds. If you put a vacuum gauge on the system and go for a drive you can watch it. Each pump will consume 4-5 inches and the pedal will feel hard at about 7-8 inches if I recall correctly. From hard pedal back to 20 inches can take up to 15 seconds depending on engine speed.