Brake pressure disappears after starting the car.
- Thread starter Sander80
- Start date
I don't know if this valve gizmo is in our (drum) MC's, or if it's the cause of the various issues that crop up in these brake threads, - so for your viewing pleasure:
Remove residual valve or not?
Hello all! As i prepare for the front disc brake conversion, I am trying to figure out the residual valve "debate". I have a 1971 800B with the stock, 2 chamber master cylinder. I have read, and been told, that the stock MC will work fine. So, I am going to try to stick with my stock unit...
www.binderplanet.com
Just Carbs
Senior Member
On disc brake cars the little valves in the master prevent the rubber dirt seals on calipers from pulling the caliper pistons to far away from the brake pads.
The rubber dirt seals are what pull the pad away from the disc when the pedal is released so as not to have excess pad dragging.
Without the restrictor valves the pad would be pulled back way to far and the take up time when the pedal is pushed would be excessive.
The rubber dirt seals are what pull the pad away from the disc when the pedal is released so as not to have excess pad dragging.
Without the restrictor valves the pad would be pulled back way to far and the take up time when the pedal is pushed would be excessive.
Assie_Phoenix
New Member
So many comments already, but just to add my thoughts, I've done this conversion on my Fury ( Dodge Phoenix here in Australia ) and I found the PCV valve was faulty and causing it to loose vacuum when I needed it. I repaired with one from a Nissan Skyline, it may have been slightly smaller but keeps the vacuum up to the dual brake booster. Also, when you finally get it working, you'll need to install a 10psi residual pressure valve into at least the front circuit and perhaps both. This will give you better pedal feel as it will keep the shoes snug up to the drums. Cheers.
Just remember that the OP has 4-wheel drum brakes.
Regarding the RPV and disk brakes, everything I've read says that 2 PSI is the standard value, they are now making 5 PSI for cars that do a lot of off-roading where there's a lot of suspension flexing. 10 PSI seems excessive and likely to lead to overheated pads/rotors. Have you had a front wheel off the ground with the 10 PSI kicking in, and tried to turn it by hand?
The guy (in the video) would not / should not be having a problem with rear drum brakes dragging. Doesn't matter if there's an RPV there or not, doesn't matter what the pressure set point is of the RPV. Because the drum brake return springs are always going to be stronger than any RPV. If rear drum shoes really are dragging, it's probably because of a problem with parking brake mechanism or brake pedal return spring.
Just Carbs
Senior Member
Or crusty wheel/caliper cylinder.
We would think that the drum return springs would always be stronger than the valve. But that has never been the case for me. Apparently 10 psi can exert a lot of pressure on a 1-inch wheel cylinder.
I too have had disc drag on a drum valve.
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Assie_Phoenix
New Member
I haven't had any trouble with 10 psi RPV using drums all round, for disks 2 psi is recommended.
Assie_Phoenix
New Member
I had dragging rear drums at one time also, but I found the springs were mounted incorrectly causing the drums to pull to the rear and engaging the shoes slightly.
Just Carbs
Senior Member
I run front disc with NO valve on the master. They did drag too much for my liking with the 2psi valve. Works fine on the 64. No negative effects now. Running 78 Cordoba 11.75 disc, calipers, spindles on that car.
Still have the 10 psi on the rear drum well, works fine. Got to have it, or there will be 2 inches of soggy pedal travel on application.
Still have the 10 psi on the rear drum well, works fine. Got to have it, or there will be 2 inches of soggy pedal travel on application.
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