[Stripes]

On our 1969 Firebird, when bleeding the brakes with a pressure bleeder(at the master cylinder with 10 lbs) I get a great volume of fluid from the front calipers, but barely any fluid from the rear wheel cylinders. Pushing the brake pedal isn't any better, barely a trickle of fluid from the bleeders on either side.

I checked the round brake delay valve (the one with the button in the rear, mounted on the MC) and it appears that the button is seized. However, that valve only delays the application of the front brakes. Does anyone have any suggestions why the rear brake fluid volume is so low? Master cylinder or rear brake hose? I'll replace the delay valve and rear hose tonight and see if it affects the rear, but I can't see how, as they are separate systems.
Any suggestions?

[gtospieg]

I think you're on the right track....rear brake hose is a possibility if it's the original or very old

[JEC3039]

Can you post a picture of the valve? I used to work at Kelsey-Hayes and some of our valves had a plunger that had to be held off when bleeding the rear systems (it opens up ports inside the valve for unrestricted flow).

You could also check the service manual specific to your car to see if it shows the special clip that was used for this.

[Stripes]

This valve has the orifice closed while the brakes are not pressed. There is no addition range to press the button in further as its as far as it goes. When you press the brake, the button pushes out, not inward. So, forget pressing the button to assist bleeding the brakes. The valve will open when the brake petal is pressed, the button moves outward.

[Stripes]

That button typically has a rubber cover over it. Mine was wet under the cover, I had a low hard petal. After repair, the petal had more range and was consistent.

[HWYSTR455]

If you're not doing a factory correct resto, you can use a later type master and eliminate the hold off valve all together. Generally, they are only needed when the master is below the bleed valves on any of the corners.


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[Stripes]

Mine delays the application of the front brakes to allow the rear brakes to apply first. It is essentially a spring-loaded piston mounted in line with the front brake line that expands when the brakes are applied. It was used on 67-69 F bodies with V-8 and or AC.

[HWYSTR455]

Some insist on using hold off valves, but pretty sure when it comes up, I comment on removing them. I guess some want the 'original' look or don't want to deal with changing brake line routing.

They basically are only for situations where a bleed valve on one of the 4 corners is mounted lower or close to the same level/plane as the master.

Sometimes due to the type of drum brake adjusters too.

If doing a system from scratch or upgrading, it's always suggested to be removed.

If you're troubleshooting a system and it's the last thing you haven't replaced, just eliminate it.


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[Sirrotica]

You could eliminate the hold off valve, then replace it with an adjustable proportioning valve.

https://www.speedwaymotors.com/Speedway-Black-Adjustable-Brake-Proportioning-Valve,1977.html?sku=91031355&utm_matchtype={match_ type}&msclkid=307685afb47810d98feee3897c016650

I have also had this happen on a 1969 GP many years ago. I blew compressed air from one of the wheel cylinder bleed screws back towards the master cylinder with the cover removed, after I emptied the reservoir. I was able to gravity bleed the rear after doing this. The problem arose when trying to pressure bleed the rear, as long as you just let it drip, the rear bled fine.

It was decades ago, 1980 ish, so details are kinda hard to recollect, but I spent hours trying to get fluid, before applying air to the bleeders. Could have been sediment, or a sticky reset button, but the air going backwards got it open.

[HWYSTR455]

The newer combo valves you don't have to push any buttons or anything, and they are pretty reliable. And if you go from disc/drum to disc/disc, you only have to change the combo valve. (Maybe the master bore size too, but no biggie.)

I've gone to adjustables again recently but used combos for years. The adjustable ones you pretty much set them and forget them, and maybe do a half to full turn when autocrossing or something. They are a little more expensive, and it's nice to dial them all the way when you're bleeding.

Many places make 'conversion' brackets and line sets so you can make combos look factory if you want, it's cleaner looking too.


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