Hi Guys, Can anyone explain how the "brake booster check valve" (p/n 101538 on US QV) works...and/or how to tell if it isn't working? Of course, I get what check valves do, but this one baffles me. The direction-of-flow arrow points to the plenum, which should mean it's open under vacuum, no? If so, the only time it's "closed" is when the engine is off....which kind of negates the need for a check valve, no? TIA for your insight!
Yes, it makes sure there is always vacuum in the brake line. It sucks out any air when throttle is closed, but closes the valve and maintains vacuum at full throttle.
Thanks. But isn't that contradictory? As long as the engine is running, vacuum would be pulled. I'm not understanding how it closes the valve when the flow points to the vacuum source. Maybe more pertinent is...how would you know if it wasn't working correctly...or getting stuck?
If it gets stuck open, the brake pedal would be stiffer, harder to brake at high revs (low vacuum). Stuck closed would be always hard to brake.
Air goes one way, vacuum goes the opposite way. As an EE, I am used to thinking electrons go one way and "holes" go the other way.
The check valve holds vacuum for the power brake booster. Max vacuum occurs during deceleration (while in gear). You still have good vacuum at idle and at cruise. Around 12-14 inches at idle. You will have almost 0 vacuum at WOT. Without the check valve your power brakes will not work well. These valves often stick open or closed. You can usually clean them with brake cleaner if they stick. I had to replace mine, it would' clean. So they can go bad. Good luck.
No, at low rpm and large throttle opening, the intake vacuum falls. conan explained it correctly; however, the check valve can also get stuck closed (so no vacuum ever reaches the brake booster). Easy enough to test -- just remove it -- you should be able to blow thru it in the direction of the arrow, and not be able to blow thru it against the arrow.
I think i'm getting it...the valve is designed to pass vacuum at a threshold value, and remain closed when vacuum drops below that? If the valve were stuck open, could that cause a stall on decel?
Yes. So if there is high vacuum in the plenum and low vacuum in the brake line, air flows from the brake line to the plenum thereby creating more vacuum in the brake line.
The check valve means the booster remains under vacuum when the engine is off. Step on the clutch on a hill when the engine is not running and you're gonna need it. Think of it like an accumulator.
AFAIK, no one has identified an inexpensive, direct aftermarket replacement -- but there probably is something out there that would work as many cars use one. Try an internet search on "power brake booster check valve" or something similar. The Ferrari PNs are 101538 and 128435 (but the price is the typical crazy ).
It's for the time when you press in the clutch with the engine not started and start rolling down the driveway and then step on the brake. It basically makes the brake booster a temporary accumulator so it will have a charge when you do that. You'll still get brakes if it fails but you will have to step on them real hard to stop rolling down the driveway.
I was looking at bake booster valves and came across this on Amazon $7.99 for two.., they look similar to the one I have in the 308 and the dimensions seem right... I might order them and try them out...: Acxico 2Pcs Brake Servo Booster No Return Inline Check Valve Vacuum Hose 10mm 1 Way Check Valve https://www.amazon.com/Acxico-Booster-Return-Inline-Vacuum/dp/B0859XTN45?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&psc=1&smid=A1OE7ERI7IZOIY
Functionally, that looks fine. The only difference might be the OD of the fittings (10mm) seems a little small -- IIRC, the ID of the stock brake booster hose is more like 12mm (but I could be wrong).
No power assist when the motor isn't running is the tipoff. And there's no Ferrari red magic in a yellow box stuff here, pretty much any check valve from any car with vacuum assist will work if you can fit it to your hoses.