delta_p
OBS Chevrolet, When Silverado Was A Trim
That's great. I think that is a much safer set up. Keep us posted on when it is finished.
Jegs proportioning valve concerns
- Thread starter Chris253
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delta_p
OBS Chevrolet, When Silverado Was A Trim
One other thing. You should probably think about bleeding this valve.
They make a combination valve bleeding tool that installs in place of the brake brake safety switch during the bleeding process. This will hold the internal piston in place during the bleeding and prevent the piston from shifting over to one side or the other due to air moving through and block the passage to the rear or front which ever direction it moved. The tool is rigid, unlike the switch which can allow the piston to move. The switch gets installed last after the bleeding. the tool looks like this
https://www.amazon.com/Team-Perform...qid=1580957281&sourceid=Mozilla-search&sr=8-4
Second. The front brakes get fluid through a metering valve. This valve takes a certain pressure before it opens to the front calipers. To prevent this and make the bleeding easier, they make a tool that will depress depressor on the end of valve and hold the valve open while you bleed. Fluid won't exit at the depressor it just holds the valve internal open. you can use anything available to hold open the valve you don't necessarily need the tool. You could probably make it. it looks like this
https://www.toolsource.com/abs-test...sor-for-abs-brake-systems-j39177-p-61417.html
They make a combination valve bleeding tool that installs in place of the brake brake safety switch during the bleeding process. This will hold the internal piston in place during the bleeding and prevent the piston from shifting over to one side or the other due to air moving through and block the passage to the rear or front which ever direction it moved. The tool is rigid, unlike the switch which can allow the piston to move. The switch gets installed last after the bleeding. the tool looks like this
https://www.amazon.com/Team-Perform...qid=1580957281&sourceid=Mozilla-search&sr=8-4
Second. The front brakes get fluid through a metering valve. This valve takes a certain pressure before it opens to the front calipers. To prevent this and make the bleeding easier, they make a tool that will depress depressor on the end of valve and hold the valve open while you bleed. Fluid won't exit at the depressor it just holds the valve internal open. you can use anything available to hold open the valve you don't necessarily need the tool. You could probably make it. it looks like this
https://www.toolsource.com/abs-test...sor-for-abs-brake-systems-j39177-p-61417.html
delta_p
OBS Chevrolet, When Silverado Was A Trim
Actually, after looking at the valve cross section. I am not sure if the depressor is actually holding the metering valve open for bleeding, or holding the metering valve in place to prevent motion of the safety piston. Either way you don't want the safety piston to move on you or it'll block the flow for bleeding and you'll be stuck trying to raise the pressure on the opposite side to get it to move back again.
Put a continuity meter on the brake switch if you try to use the depressor alone to bleed.
Put a continuity meter on the brake switch if you try to use the depressor alone to bleed.
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Total waste of time, money, effort, and enthusiasm.
Every GM safety switch I've worked with is spring-loaded to the neutral position. Remove the pressure imbalance, and they reset themselves.
They don't block fluid flow. They just signal a light on the dashboard.
I have worked on two Fords that I had to manually re-set the safety switch. They didn't block fluid either, but they had to be manually returned to the neutral (light off) position.
The metering valve is open at very low pressure. It closes somewhere ABOVE 15--20 psi, then opens again at higher pressure (~100 psi?) so that the front brakes work.
DEPENDING on your bleeding method, either you won't have enough pressure to close the valve, or you'll have so much it'll pop open again. The "special tool" is used when a pressure bleeder having more than 20-something psi, but less than 100 psi is connected.
In decades of bleeding brakes, I've NEVER needed to use a special tool to keep the metering valve in a fluid-flowing position. It's just never been an issue. True enough, I generally tend to gravity bleed (not enough pressure to close the valve) or I use a pressure bleeder with ~15 psi in the tank (again, not enough pressure to close the metering valve) or I have someone pump the pedal (more than enough pressure to re-open the metering valve.)
delta_p
OBS Chevrolet, When Silverado Was A Trim
That's good info.
The manual of the '94 though does say the lamp switch can only be reset by fixing the malfunction (assuming this means leak) and applying pedal force to generate at least 450 psi. Or manually i suppose.
It doesn't look like the '94 valve above has a spring on that warning piston. If it were me, I'd hold the piston with the brake light tool during bleeding.
And it doesn't say the piston blocks the flow like you said as well. It just says the safety aspect is built into the valve. I assume it is in both of the two valves at the front and rear. If you know how it does it, please share. I am always interested in learning this. This particular valve looks like the farrari version of the older style I am used to.
I do think the OP made the right decision in this method of ABS delete, hope he does a write up, and whatever writeup that told him to T the front to the master cylinder gets trashed.
The manual of the '94 though does say the lamp switch can only be reset by fixing the malfunction (assuming this means leak) and applying pedal force to generate at least 450 psi. Or manually i suppose.
It doesn't look like the '94 valve above has a spring on that warning piston. If it were me, I'd hold the piston with the brake light tool during bleeding.
And it doesn't say the piston blocks the flow like you said as well. It just says the safety aspect is built into the valve. I assume it is in both of the two valves at the front and rear. If you know how it does it, please share. I am always interested in learning this. This particular valve looks like the farrari version of the older style I am used to.
I do think the OP made the right decision in this method of ABS delete, hope he does a write up, and whatever writeup that told him to T the front to the master cylinder gets trashed.
I'm a little surprised at that.
When I meant "manually reset", the Fords I did had to have the hydraulic system opened--a bleeder screw is easiest--on the side that DIDN'T leak, and then press the pedal. This creates an "opposite" hydraulic "failure" that pushes the safety-switch piston back to the center position. 'Course, if you go too far...or not far enough, not only does the light stay on, but you've lost track of where the piston is, so then you're dicking with bleeder valves on both circuits trying to get the piston centered.
I guess I wasn't smart enough to take the switch OUT, and look at the position of the safety-switch piston.
The spring could be in the switch assembly, riding on the tapered section of the piston.
The "safety aspect" is that it turns a light on warning you that half the hydraulic system has failed. It doesn't stop the leak. The leak stops when the master cylinder reservoir goes dry, or someone fixes the problem.
delta_p
OBS Chevrolet, When Silverado Was A Trim
The safety aspect I was mention is the one that will allow full pressure to the front brakes if the rears fail a leak or full pressure to the rear if the front brakes fail a leak. Normally the combi valves I have seen the same piston that the brake light is on shuttled towards the low pressure side that leaks and shuts off flow to that side so the other side gets all the pressure and the driver still has some brakes to be safe.
For instance the manual also states the combination valve has a by-pass feature that ensures full system pressure to the rear brakes in the event of a front brake malfunction or full system pressure to the front brakes if the rear brakes malfunction.
This is really one of the top reasons for using a combination valve to me. I can't work out how the valve shown from a '94 actually accomplishes this.
This is type i am used to
For instance the manual also states the combination valve has a by-pass feature that ensures full system pressure to the rear brakes in the event of a front brake malfunction or full system pressure to the front brakes if the rear brakes malfunction.
This is really one of the top reasons for using a combination valve to me. I can't work out how the valve shown from a '94 actually accomplishes this.
This is type i am used to
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The two hydraulic systems are entirely separate except at the master cylinder, and--depending on your perspective--at the safety switch piston.
Keeping one side functional when there's a failure on the other side isn't done by the safety switch piston; or by the combo valve at all.
The safety switch piston "senses" pressure on both hydraulic circuits, but the O-ring seals prevent pressure loss from one hydraulic system to the other. The hydraulic circuits cannot "mix". Note that this is not the same as preventing fluid loss on the side that has a leak, which the safety valve piston does NOT do. As a result, THE MASTER CYLINDER RESERVOIR IS DIVIDED, so a fluid leak on one hydraulic circuit can't empty the reservoir for the other hydraulic circuit.--but it can empty the reservoir half on the side that does leak!
NORMALLY, you press on the brake pedal, you physically move the master cylinder primary piston via linkage (even with a power-brake booster in place) The primary piston can be connected to either the front or the rear brakes, or one side of a diagonal system, or some other method of splitting the hydraulic systems. It's been done multiple ways.
IF (big IF) there's no leakage of fluid, HYDRAULIC pressure generated by the primary piston (with a little help from a fairly-weak spring) moves the secondary piston. The secondary will be connected to the "other" brakes.
If there's a leak in the primary circuit, there's a projection on the primary piston that will mechanically move the secondary. Whatever brakes the primary piston controls will be weak or useless, the brake pedal will be very low--but the secondary piston is forced to move, so that whatever brakes the secondary activates still work.
If there's a leak in the secondary circuit, there's a projection on the secondary piston that contacts the end of the master cylinder bore. The brakes activated by the secondary circuit will be weak or useless, but when the secondary piston contacts the end of the bore, the primary circuit begins to build pressure which activates the brakes in the primary hydraulic circuit. Again, the brake pedal will be very low.
It's the MASTER CYLINDER not the combo valve that assures that the non-leaking hydraulic circuit still functions. (Although the seals on the safety switch piston assure that the high-pressure system doesn't bleed into the leaking, low-pressure system.)
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delta_p
OBS Chevrolet, When Silverado Was A Trim
Yes I understand how the master cylinder works and in response to a pressure loss on one circuit. I guess I am saying that's not the only feature in the system performing a response to a loss in pressure. I believe combination valve to also aid in forcing pressure to the opposite circuit. This is the only time I have seen in writtings or otherwise to negate that. I am not saying this is wrong, just that I haven't come to that conclusion yet.
Reactions to loss of pressure in one circuit:
Master cylinder as described previously.
Safety switch piston position--which turns on the dash light but doesn't prevent fluid loss or block a hydraulic passage.
Fluid on the chassis or on the ground or on the friction linings if the leak is external instead of internal.
Low brake pedal
Poor stopping ability due to reduced brake effectiveness.
Crash, boom, bang. Big squirt of adrenaline.
Money spent fixing the car is not available for Rent or beer or **** or whatever.
Yeah, there's lots of stuff that "responds" to a loss of pressure.
Not possible. There's nothing in the combo valve that creates pressure; or allows pressure to transfer from one circuit to another. Both the metering valve and the proportioning valve REDUCE pressure to their respective brake cylinders under some conditions. They never increase pressure above what the master cylinder produces.
Not desireable: Transfering pressure from the "good" circuit to the leaking circuit, would rob pressure needed for the brakes that work, would hasten fluid loss, and would deplete both master cylinder reservoirs. Of course, you can't transfer pressure from the low-pressure leaking side to the high-pressure side. That goes back to "Not possible", above.
The closest we get to pressure transfer is the skinny lil' piston shifting a quarter-inch (or less) towards the side of low pressure, and the infinitesimal pressure increase from that goes out the leak that caused the low pressure to begin with.
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