[kyle_blake]

hi,

my stock 69 gto that came with front disc and rear drum from factory kept having my brake pedal go soft after repeated gravity bleed. would work for 2-3 drives then soft. After many mechanics, and no real direction, I finally looked behind rubber button on hold off valve and brake fluid was there, so i bought a replacement valve.

After a very long time of replacing everything else everywhere and bypassing the valve
i believe i experienced symptoms of residual pressure always pushing down the front brake pads as i drove around and heating up the rotors 135 F . maybe it would go up more if i kept driving. i do not know norms.

Oh and once a shop a loosened the bleeder valve on caliper after driving for 50 mins and the fluid "shot out" of it like pressure had been built up? just sharing some what shop told me. that is not normal. all my old brake posts are are around on py. figured i'd start new.

I installed the valve yesterday and went to manually bleed the brakes using 2 man method and it doesn't seem to want to push fluid past out of the valve down to front brakes. i tried about 9 presses. i did not try it with car started doing that.


Just wondering if anyone has had same experience? Clearly I can not gravity bleed it. It is the PV68 model number. right stuff i believe makes it.

I have a friend with a pressure bleeder to try. i believe it can go over 15 psi down from master cylinder to all 4 tires.

Any comments would be appreciated, thanks so much... happy pre green drinking st patricks day coming up.

ps - there is a rubber button on backside of part that one can press which moves the valve inside but i forgot what that is for?

thanks

[Formulas]

Crack open the line connection right before your rubber lines to verify yes or no getting past your new valve... ... A couple things high on the list of brake hold on or not working is collapsed rubber flex lines and sticky caliper pucks

[scott70]

My 70 gto has the valve right under the master cylinder. I have to push that button during bleeding. I believe GM actually had a tool to do it but I remember right I wedged a chisel between the button and the power brake booster to hold it in.

[grivera]

As Scott mentioned, the button has to be pressed constantly during the front brakes bleeding process. I bled the brakes on my 69 Lemans using a pressure bleeder and you could easily see the difference in flow with button pressed vs not pressed.

[scott70]

Also as you probably know that rubber is just a protector… the actual button is small and metal underneath the rubber.

[kyle_blake]

Thanks very much. i'll try and push the button that pushes the cylinder valve up then re-try bleeding / gravity. thank you again everyone.

[kyle_blake]

I have the pv68 from "right stuff" and tech support did say you have to press the button to get it to allow fluid. I was able to do that and it allowed fluid through. Funny thing I had initially messaged "inline tube" before I looked at invoice, i thought the item would of been the same!

Inline said said you do not have to press the button to bleed. Go figure?!?!?!
Their item is :
https://www.inlinetube.com/products/pr105

maybe this helps someone in the future. so thought I'd update.

have a good day

[Schurkey]

The rubber cover on the combination valve is to protect the stem of the holdoff/metering valve.

The holdoff/metering valve is a three-way deal.
1. At zero-to-low-pressure, it vents so that when temperature change expands the brake fluid, it can push backwards into the master cylinder, and from there up to the reservoir via the ports between reservoir and cylinder.

2. At moderate pressure, the valve closes so that the brake pressure in the rear hydraulic circuit is high enough to overcome the brake shoe return springs before the front brakes engage.

3. At normal braking pressure and above, the valve opens again so that the front brakes apply.

The ONLY time that valve is supposed to be closed is when the hydraulic pressure is building-up from "none" to whatever it takes to overcome return-spring pressure at the rear wheel cylinders.


If you are gravity-bleeding, there's essentially no pressure in the hydraulic system, the metering valve allows fluid flow to the front brakes.
If you are vacuum-bleeding, there's essentially no pressure in the hydraulic system, the metering valve allows fluid flow to the front brakes.
If you are two-person, pump-the-pedal bleeding, you have so much pressure in the hydraulic system that the valve is blown open, allowing fluid flow to the front brakes.

So the ONLY time you'd need to dick with that valve is when pressure bleeding at the moderate pressure the metering/holdoff valve is intended to close at. There are "special tools" intended to block that valve stem "open" to allow fluid flow to the front brakes.

For the record, forty-plus years of bleeding brakes, and I have NEVER EVER needed to dick with the metering valve stem; including when I connect my pressure bleeder at 10--15 psi. I NEVER have problems getting fluid flow to the front brakes. When I have played with the metering valve stem, it has turned out that the front brake hoses were plugged--swollen shut.



If you have retained pressure in the front hydraulic system of a disc-brake vehicle:
1. The aftermarket metering valve is a piece of defective crap, or
2. The master cylinder pistons aren't returning to their "home" or "retracted" positions--perhaps due to incorrect pedal-to-booster pushrod adjustment, incorrect booster-to-master cylinder pushrod adjustment, incorrect pedal travel--misadjusted brake light switch, missing return spring, incorrect master cylinder, etc., or
3. Restricted brake hoses. Far more common than most folks suspect, or
4. Grossly-overfilled brake fluid reservoir, when the fluid gets warm and expands, it applies pressure to the whole hydraulic system.

Sections 2-23 and 2-24, figure 2-12:

[kyle_blake]

Hi Schurkey,

Thanks for detailed response I always look forward to your posts! Just curious the picture you posted is for a later style combination valve that is an "all in one piece".

Which I believe is not the valve I am referring to? Mine sits under the master cylinder stock for the 69 gto. Unless I misunderstood your post?

Mine has the distribution block near drivers side frame rail that simply splits the brake fluid front/back with the brake light warning switch. This valve accepts front disk brake fluid and then routes it down to the distribution block.

I posted a picture of it. I have yet to use a pressure bleeder to do all 4 and complete this task.

Thanks again for taking the time to respond, as always I really appreciate PY and it's users.

[scott70]

It’s the hold off valve right under the master cylinder. I’ve always had to have the button depressed while bleeding the front brakes and a quick google search shows over and over that the button needs to be held in while bleeding.

[Sirrotica]

If you center the stem/spool on the valve to the neutral position, then let the brakes gravity bleed, there is no pressure to push the spool over center shutting off half of the system. Both circuits are open, and gravity and barometric pressure will do the bulk of the work for you.

Having worked alone most of the time I've been twisting wrenches I needed to find a way to bleed brakes without any assistance. My current wife has been with me for 20 years, and she doesn't have a clue on how to sit in the passenger compartment, and pump brakes for me, because I've never asked her to.

My father showed me how to gravity bleed brakes when I was 12 YO, been doing it that way for 60 years......... I've never used any special tools, a pressure bleeder, nor used a vacuum pump to bleed brakes, just used gravity, and physics, KISS principle.

[Schurkey]

The holdoff/metering valve can be separate, or integrated into a "combination valve". Either design works the same way.