What ARE these? (1 Viewer)

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My toyota manual says these are "check valves".

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My understanding of a "check valve" is that it is a "non-return valve". (In other words it allows a flow in one direction but not the other.)

If they were truly "check valves" isn't it true that the brakes would never release? (The valves would allow the fluid to flow from the master cylinder into the wheel cylinders when the brakes are applied but then prevent this fluid from returning to reservoir afterwards!)

So has toyota labelled them wrong? And if so, what is their real purpose?

Any help would be greatly appreciated.
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I think the term used most commonly in this forum is "residual valves". Kinda like a check valve but only holds a certain amount of pressure. With drum brakes they are for the purpose of holding enough pressure so the springs don't push back the wheel cylinders and make you have to pump the brakes multiple times to get a firm pedal each time you apply the brakes.
 
do you mean the brake pressure switches ?
 
I think the term used most commonly in this forum is "residual valves". Kinda like a check valve but only holds a certain amount of pressure. With drum brakes they are for the purpose of holding enough pressure so the springs don't push back the wheel cylinders and make you have to pump the brakes multiple times to get a firm pedal each time you apply the brakes.

Thanks Dgangle.

I've been reading up other posts and thought it must be something like that.

But it still doesn't make complete sense to me because I thought it was the brake adjustors on the wheels that did that job (stopping the wheel-cylinder-pistons from having to move too far)?

If I don't get an answer I fully understand from this thread, I may just take them apart and see for myself what's inside?
 
There are pressure valves in the master cylinder that keeps the springs from pulling the slave cylinders all the way back to a "rest" position. The purpose is that, if your brakes are not properly bled and/or adjusted, the brakes will stay partway extended through the pumping to add distance from "rest" until you're able to get a firm pedal.

Clear as mud?
 
There are pressure valves in the master cylinder that keeps the springs from pulling the slave cylinders all the way back to a "rest" position. The purpose is that, if your brakes are not properly bled and/or adjusted, the brakes will stay partway extended through the pumping to add distance from "rest" until you're able to get a firm pedal.
Clear as mud?

Ah thanks projektdotnet. You have done well by getting through my thick skull.

So to put it in my words, these "valves" provide "free-flow" for fluid exiting the mastercylinder but "restricted flow" for fluid that is returning. - Doing this, enables a driver to still achieve some braking effect on all wheels (through "pumping the brakes") even with "air in the brake lines".

So "residual pressure valves" is a better term than "check valves" but even this term is confusing because they only leave the RESIDUAL pressure FOR A BRIEF TIME. (Sufficient for the next downstroke (during pedal-pumping) to add to it before it is lost.)

Thanks again. Gosh MUD is useful.

(You can see why I asked if you view my last post in my "Do-it-yourself booster overhaul" thread. Thanks to you and the others for helping me out here.)
 
Thanks pbgbottle.

Yes. It is posts like that one that actually added somewhat to my confusion because the writer(s) appears to think that these valves leave some positive pressure PERMANENTLY in the brake line. I don't believe they do that at all. (If they did, they would promote "brake drag" - and my brakes have never done that (when they've been adjusted properly and "run in").

Cheers:)
 
The truth is confirmed

Time and time again I cop the consequences of taking shortcuts but I never seem to learn. I didn't open up those "valve thingies" when I reconditioned this mastercylinder - So now I've had to pull things off again to get at them. (I couldn't get any fluid to come out of the master cylinder at all.)

At least this should silence any debate on the purpose of these "thingies" (- As far as MY mastercylinder is concerned anyway. Perhaps others are different?)

Boy- What a mess I found inside! I expected it to be clean because I have been meticulous with fluid changing and maintenance but they couldn't have been more clogged up with "crud". I seriously think the manufacturer (AISIN) leaves all the debris from their manufacturing work in places like this. It is the only explanation I can think of for what I tend to find!

I used a 27mm socket to undo those big "hex nuts" and it took so much force I was worried about buckling the firewall. Anyway, here's a photo of the m/cyl after I removed the "valve thingies" and spent a lot of time cleaning out the crud-caked holes they sat in:
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On my "drums-all-round" BJ40, these "valve thingies" are identical for the front and rear brake circuits.

They don't leave any residual pressure permanently in any line because they have one orifice that NEVER CLOSES.

They do however allow 5 times the OUTFLOW compared to INFLOW because of four extra "valved side-ports" that open only for outflow. ------And this provides the safety feature that projekdotnet helped me to understand. (The ability to successfully "pump the brakes" when you have air in the lines.)

I suspect those of us that discard these "valve thingies" are also discarding this valuable safety feature.

There is also a feature that allows unrestricted backflow when the backflow pressure exceeds the spring pressure. I'm not entirely sure why this is present. - Probably to ensure the "valved side-ports" don't get damaged - ----Say, -when you force a wheel cylinder piston back in with a G-clamp as I sometimes do when fitting new linings.
Anyway here is what each "valve thingie" actually looks like on my m/cylinder.
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the valves are usually only discarded when swapping to a disc brake setup, discs just work differently, if you leave these in place it pumps up and locks your brakes on. there was some discussion of a lower pressure residual valve on disc setups, but IIRC, most don't have them and have no issues.
 
the valves are usually only discarded when swapping to a disc brake setup, discs just work differently, if you leave these in place it pumps up and locks your brakes on. there was some discussion of a lower pressure residual valve on disc setups, but IIRC, most don't have them and have no issues.

But the fact that these "thingies" CAN'T leave any permanent pressure at all in a brake line contradicts what you are saying.

I thought the photos would prove that they can't leave LASTING residual pressure?

All I can think of to explain what you and others are saying is that the "valve thingies" you are removing are entirely different to mine ---and that they can somehow be selected to leave differing amounts of PERMANENT residual pressure (like 1psi, 2psi etc)
 
that is what our U.S. 1975ish with 4 wheel drum brakes ,residual valves also look like:grinpimp:

Thanks pbgbottle. Well they certainly look identical to what I found!
 
Yes they do hold pressure on about 10 psi on drum brakes usually about 2 on disks. You can read from the manuf or google some hot rod builds they talk this---I mean ol rods T buckets and that sort.

This link explains it pretty well they sell adjustable residual valves so you can get the pressure you want.

According to this one effect of eliminating them could be that you have to push the brakes more than once to get good braking---this doesn't necessarily mean it would happen everytime--but Murphy's law applies! You're doing the right thing by asking--safety first

I think Lawyers and Insurance companies could have a field day if you modify your brakes and don't have some pretty good answers.

On the other hand i don't see anything wrong with properly installed aftermarket versions adjusted to the correct pressure setting for your disc and or drum configuration.





http://www.sachserodshop.com/pdf/55.pdf

Brake Residual Pressure Valves​
Residual pressure valves hold a preset pressure from the valve to the brakes be it disc or drum.
On some cars with the master cylinder lower than the brakes on each wheel, by holding a set
pressure it insures that the brake fluid doe’s not siphon back into the master cylinder. Also as
with the case of drum brakes the return springs installed on the brake shoes will collapse the slave cylinder where you
would not have contact from the shoes to the drums with just one push on the pedal. With 10 psi residual pressure
held, the shoes are held close to the drums for instant braking on the first push of the pedal.
Disc Brakes ......................................... 2lb. Residual Valve .......................................... SSB A0764 .......... $19.95
Drum Brakes .......................................10lb. Residual Valve ......................................... SSB A0765 .......... $19.95​
 
10 psi check valve

just to clarify a little -- a check valve does not have to be an all or nothing proposition it can allow some return flow up to a point


if you can imagine say a plate blocking a hole and having a spring behind it rated at 10 psi --above 10 psi and the plate is off its seat and allows flow but a below 10 psi the the spring force is greater than the hyd force and closes off the hole leaving 10 psi trapped in the line. These aren't the components exactly but that is the principle.

in some aircraft a residual valve is installed in the emergency brake lines to relieve pressure if it builds up above a certain level but to close off if full flow emergency brakes are applied--they can design things to do what they want them to do pretty much --in the case of the aircraft it will bleed off low press/flow but not high press/flow, pretty cool and it keep you from landing with teh emer brakes applied!
 
Yes they do hold pressure on about 10 psi on drum brakes usually about 2 on disks.
Brake Residual Pressure Valves
Residual pressure valves hold a preset pressure from the valve to the brakes be it disc or drum.

Well at least I now see where the information has originated from. -Thanks for that.

This being true, what I have on my ORIGINAL Aisin mastercylinder (the one the vehicle left the factory with) are not "Residual Pressure Valves" at all.
 
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if you can imagine say a plate blocking a hole and having a spring behind it rated at 10 psi --above 10 psi and the plate is off its seat and allows flow but a below 10 psi the the spring force is greater than the hyd force and closes off the hole leaving 10 psi trapped in the line. These aren't the components exactly but that is the principle.

Thanks bsmith. Can I suggest you take another look at my photos though. (Of the "valve thingie").

The hole in the centre of the cup is OPEN ALL THE TIME. Nothing EVER moves to block it. This hole makes it IMPOSSIBLE for any residual pressure to be left in my brake lines (other than from the weight of the fluid itself) - provided no-one is touching the brake pedal of course.

Edit Nov 09 - Since this thread was posted - my understanding has changed. I now believe these "residual valves" are fitted to maintain a slight positive pressure in the brake lines BUT ONLY WHILE THE WHEEL-CYLINDER-PISTONS ARE RETURNING TO THEIR NORMAL POSTIONS AFTER A "BRAKE APPLICATION". I think this is done to keep pressure on the lips of the seals to help avoid fluid leakage. (Not so important on disc callipers because their pistons move far less.)
 
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