I had issues with my stock brake master cylinder in my 47 Kaiser so i decided to change it out to a dual resivor drum to drum 1969 chevy truck master cylinder. i mounted it on the firewall and made all new brake lines. I bleed them with a mighty vac and the old fashion way with a buddy. The problem is they will pump up and hold pressure just fine but after you let off for a few seconds they drop to the floor and have to pump them up again. My question is do i need to run a residule pressure valve or something?
Gravity bleed them. Don't touch the pedal, you are making a large bubble in to many small bubbles. They "gel" again and cause the lack of pedal.
Fill the master, leave the cap off, crack all the bleeders and let it drip, keep an eye on the master and don't let it empty. Seal it up, one really slow push on the pedal and do it again. I mean really really really slow. Clean up and enjoy.
It's possible that the brakes are way out of adjustment and you have pump it up to take up the slack. Adjust the brakes and see what you have.
And going back to your original question. a drum brake system requires residual pressure to maintain wheel cylinder cup seal. A dual drum/ drum system should have a 10psi residual check valve in each circuit, usually found in the outports of the master cylinder.
The pedal height would come up from adjusting the shoes closer to the drums, but that won't cause the pedal to be at the bottom after sitting.
......and make sure you put those check valves in also ,once the bleeding issues are sorted. I used Wilwood, clean the annodise coating off , if ya dont like that look .hahah
Did you put thread tape on the threads of the bleeder valve? It will stop air from going back into the cylinder when using the Mighty Vac. I think it's an air bubble or brake adjustment issue.
Brake adjustment is more critical on cars with 4 W drums like a Kaiser than the modern disc/drum set ups that we are used to. It still could be air in the lines but start by adjusting all the pedal slop out of the system. Air in the system will give a spongy pedal because the air bubbles will compress giving you that feel. A pedal that pumps up to a high hard pedal is a sign of a possible adjustment problem.
the master cylinder might be bad.i had one with an internal leak and it would bypass itself.punp it um acouple of times then it would work great then let it sit for a minute then the same thing no brake pedal.just a thought.
I like to run a line off of the bleeder into a jar of brake fluid. pump it a couple times, it blows air out and sucks fluid in. works when you are by yourself. I've gotten some bad masters from the store as well. bench bleeding the master works. don't know why, but it really is necessary.
I had a friend explain bench bleeding the m/c to me in a way I could understand it the other day. I could not figure out how it worked when you bench bled it then took it to the car to mount it how it would stay bled. He told me it was like holding a half full pop bottle up side down where you can see the liquid staying in the bottle. My pee brain could understand that. Also, correct me if I'm wrong, but because he has the m/c on the firewall higher than the wheel cylinders, I don't think RPV's are needed.
OK, I will correct you. RPV's are not required in a DISC brake system when the master is mounted above the calipers. A 2psi RPV is required on a disc system when the master is low mounted to prevent fluid drainback. 10psi RPV's are always required in a drum brake system to maintain an air tight seal of the wheel cylinder cups during brake release. The only exception would be some late model wheel cylinder cups which incorporate an expander spring inside the cups.
BUT... since you used a drum/drum m/c, the RPV's may be built into the m/c ALREADY. Ask me how I know...
"10psi RPV's are always required in a drum brake system to maintain an air tight seal of the wheel cylinder cups during brake release." Really?! I wonder how all the old cars had brakes then before the advent of RPV's.
This info from ECI: [FONT=Verdana, Arial, Helvetica]Residual Pressure Valves are used in a both front and rear brake system as follows:[/FONT] [FONT=Verdana, Arial, Helvetica]2 PSI Valves[/FONT][FONT=Verdana, Arial, Helvetica] - These valves are used in a disc brake system only and are required when the master cylinder is at, or below, the height of the calipers. It's purpose is to act as an anti-siphon valve preventing the brake fluid from siphoning back into the master cylinder when the brake pedal is released. Even if the master cylinder is even or slightly above the calipers, put one in anyway. If you don't and you park on a hill, fluid will siphon! These valves are cheap insurance - put them in![/FONT] [FONT=Verdana, Arial, Helvetica]NOTE: You will know if you need one of these valves if you had to pump the pedal twice to get a good pedal. See illustration for more. [/FONT] [FONT=Verdana, Arial, Helvetica]10 PSI Valves - These valves are used in a drum brake system to prevent air from being ingested into the hydraulic system when you release the brake pedal. Typical wheel cylinder seals only seal when there is pressure behind them. Rapid release of the brake pedal creates a vacuum in the system which causes the seals to relax and air is ingested into the wheel cylinders. Maintaining 10 PSI in the system at all times prevents this. Some disc/drum master cylinders have 10 PSI residual pressure valves installed internally, some don't. If you're not sure, call us and we can tell you how to check. Also, some new style wheel cylinders have cup expanders which negate the need for the residual pressure valve. Either way, if you are not sure whether you have one or not, put one in. They are not cumulative and it won't hurt anything if you have two. Don't worry about brake drag, it takes roughly 75 PSI to overcome the return springs.[/FONT] [FONT=Verdana, Arial, Helvetica, sans-serif] [/FONT]
You didn't say what the master cylinder bore size was on the 47. Whatever the original bore size was on the 47 has to be the same bore size on the dual m/c replacement, otherwise not enough fluid volume in one stroke to make the brakes fill and function. One size does not fit all, an 1/8 inch in bore size is like night and day.
All of you had great info but whoever said go adjust your brakes was right! I went out to the shop last night adjusted everything and had a decent pedal i bleed them one more time and had a great pedal. I didnt think adjusting them would make that much of a difference but it did. Thanks for eveyones help
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