Front Brake Hold Off Valve Questions - Team Camaro Tech
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post #1 of 7 (permalink) Old Nov 24th, 16, 08:19 AM Thread Starter
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Chris
 
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Front Brake Hold Off Valve Questions

The recent thread discussing the Motive Brake Bleeder got me thinking about the hold off valve on my '69. I've never had a problem bleeding the front brakes without depressing the button on the back of the valve. I see evidence of leakage at the rear line connection of my valve, but it's the original valve so who knows when that occurred. Here are my questions:
(1) Do these valves go bad and if so how do you know?
(2) The button on the back of my valve doesn't seem to move at all when I press it. Do you actually feel it depress when you push on it? (maybe mine is stuck in the depressed position which is why its easy to bleed my front brakes)
(3) Should the button be easy to depress or is it pretty hard? (given its location, I'd hope it wouldn't be too hard or you'd never be able to push it in without unbolting the valve first)
(4) To bleed the front brakes do you press the button once or does it have to be held in the depressed position as the brakes are bled?

Thanks.

'69 Z/28, Lemans Blue/White Stripes, DZ302, Tremec Magnum, 4.10, Procar Elite Seats,
Custom Upholstery, Dynatech headers, 17" VWW V45's, 235/45, 275/40, C5 Front Brakes
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post #2 of 7 (permalink) Old Nov 24th, 16, 11:57 AM
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David Pozzi
 
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Re: Front Brake Hold Off Valve Questions

Quote:
Originally Posted by ctcz28 View Post
The recent thread discussing the Motive Brake Bleeder got me thinking about the hold off valve on my '69. I've never had a problem bleeding the front brakes without depressing the button on the back of the valve. I see evidence of leakage at the rear line connection of my valve, but it's the original valve so who knows when that occurred. Here are my questions:
(1) Do these valves go bad and if so how do you know?
(2) The button on the back of my valve doesn't seem to move at all when I press it. Do you actually feel it depress when you push on it? (maybe mine is stuck in the depressed position which is why its easy to bleed my front brakes)
(3) Should the button be easy to depress or is it pretty hard? (given its location, I'd hope it wouldn't be too hard or you'd never be able to push it in without unbolting the valve first)
(4) To bleed the front brakes do you press the button once or does it have to be held in the depressed position as the brakes are bled?

Thanks.
1 yes, they leak around the "stem" or push button under the rubber cover. There is an o ring inside.
2. 3. it should move inward and stay there, but pressure inside the line will push it back out, like stepping on the brake pedal.
4. you have to press it in and hold it. you have to clamp it in, you can't hold it with your fingers.
Once you develop more than 30 to 40 psi in the brake lines, the valve opens and allows fluid to go to the front brakes. That is enough pressure to overcome the rear brake shoe springs pressure so front and rear brakes start operating at the same time. Imagine a cold icy morning your auto choke is on high idle 2200 rpm's. you put the car in gear, auto transmission, your foot is on the brake but without the valve, light foot pressure will hold the front discs but the rear shoes will not be up against the drums. The rear wheels would spin if the car is on an icy street.
A pretty rare issue.

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post #3 of 7 (permalink) Old Nov 24th, 16, 12:29 PM
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Al
 
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Re: Front Brake Hold Off Valve Questions

Quote:
Originally Posted by davidpozzi View Post
Imagine a cold icy morning your auto choke is on high idle 2200 rpm's. you put the car in gear, auto transmission, your foot is on the brake but without the valve, light foot pressure will hold the front discs but the rear shoes will not be up against the drums. The rear wheels would spin if the car is on an icy street.
A pretty rare issue.
...which makes you wonder why manual trans cars got the valve....



.


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post #4 of 7 (permalink) Old Nov 24th, 16, 12:42 PM
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Re: Front Brake Hold Off Valve Questions

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Originally Posted by BPOS View Post
...which makes you wonder why manual trans cars got the valve....



.
Because you still want the front and rear brakes to operate at the same time, whether auto or manual.

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post #5 of 7 (permalink) Old Nov 24th, 16, 01:40 PM
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Re: Front Brake Hold Off Valve Questions

It could be for braking lightly on ice. The auto trans explanation is one I've seen quoted as coming from GM.

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67 RS 327 original owner. 1965 Lola T-70
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post #6 of 7 (permalink) Old Nov 24th, 16, 01:51 PM
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Re: Front Brake Hold Off Valve Questions

Here you can find articles on rebuilding brake valves and they sell the parts also

https://www.musclecarresearch.com/articles
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post #7 of 7 (permalink) Old Nov 25th, 16, 06:39 AM Thread Starter
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Chris
 
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Re: Front Brake Hold Off Valve Questions

Thanks to you all for the feedback, it's a huge help. Answered all my questions and even provided a source for rebuild information and parts from a new source I'd never seen referenced - fantastic! That's what I love about this site, you learn something new every day. Guess I need to rebuild my valve which also means I'll need to bleed my brakes again - I hate that job. I still wonder why I've been able to bleed the front brakes using the Motive and only about 10 - 15 psi and not depress the valve. Maybe it's pitted inside and leaking past the seals. Oh well, just one of the joys of owning these cars.

'69 Z/28, Lemans Blue/White Stripes, DZ302, Tremec Magnum, 4.10, Procar Elite Seats,
Custom Upholstery, Dynatech headers, 17" VWW V45's, 235/45, 275/40, C5 Front Brakes
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