Brakes losing pressure at red lights, any ideas?

[66fyssh]

I have a 66 Barracuda with Kelsey Hayes front discs and original rear drums. It still has the original single reservoir master cylinder. I recently rebuilt the proportioning valve that was leaking. The proportioning valve is no longer leaking and the car stops well, but now the brakes are losing pressure at stop lights. After a minute of two of keeping the pedal depressed, it starts losing pressure. One pump and it's back up.

I bled all four corners and didn't get any air and it isn't losing any fluid that I can see.

Could air be leaking out of the master cylinder cap? It looks completely original. What's the best way to make a new gasket for inside the cap?

Thanks for your help!!

 

[Kernel Sanders]

if there is no leaks in the system, my bet would be an internal leak in the master cylinder.

 

[OUTLAW]

If there isn't any external leaks then it sounds like the master cylinder is bypassing back into the reservoir. Time to rebuild/replace/or upgrade the master cylinder

 

[charger426]

Best thing would be to get a new duel reservoir master cylinder anyway....just safer and cheap insurance if you ever have a brake line go bad.

 

[KosmicKuda]

You still have a leak!

You should visually check the front calipers. the rear wheel cylinders, and every inch of the brake lines for seepage. But I would bet on the master cylinder being the culprit. A "leaking" MC cover has no effect on brake performance because the piston should remain covered by brake fluid at all times. (unless the fluid is low)

This could get scary if you keep driving on that single chamber MC ! If it fails, you are SOL. I've driven cars when one circuit of a dual system fails and it's bad. If your single circuit suddenly has a complete failure, it may be "Hello Progressive" phone call time.

If it were me, I'd convert to a 73-75 A-body power brake MC but run manually. The 7/8" bore gives a very nice feel w/o the power brakes and is far safer.

I'm all for correctly restored cars but safety should come first. I'm converting my "new" survivor 64 Valiant to a dual system before it's even drivable.

 

[J-MacsMopars]

Had this same problem with my duster just recently. Turned out to be the MC. Changed that and it stopped so much better than it had before and it holds pressure.

 

[66fyssh]

Thanks everyone. Converting the M/C to a dual has been on my list, but I guess the time is now! I like the original exterior but not concerned about under the hood being original. I already updated to a Pertronix for better ignition and will go with an HEI at some point. Also want to do the headlight relay project. Both using parts from members here!

KosmicKuda, appreciate the suggestion on 73-75 a-body power. Are there any clearance issues with that?

I'll search the forums on converting to two lines into M/C.

 

[67Dart273]

Converting is easy. They make these magical things known as "inverted flare plugs."

http://www.summitracing.com/parts/rnb-43160?seid=srese1&gclid=CMPfvsG158UCFY-TfgodJpgAZg

The trick is you will have to determine "what size" Often, at distribution points, the flare nuts are special nuts, that is, NOT the "standard" size matching the line. If they are, you will want "3/16" size

Pull the rear line coming out of your distribution block, and plug the block.

Either extend or make up a new like between the prop valve and the FRONT fitting on your new master.

Connect the REAR fitting on the new master to the old distribution block.

The two nuts for the master are likely to be special sizes. You will either have to round up nuts to fit the master.........likely different sizes front and rear........to fit 3/16 line

OR sometimes you can buy "step up" or "down" fittings.

ALL these fittings MUST be "inverted flare" No pipe thread.

 

[66fyssh]

Great info 67dart273!! It helps a lot! I've done wheel cylinders, shoes and pads, hoses, now the proportioning valve...but never had a M/C off before.

I'm looking around online now at M/C's. Probably go with Rockauto? They list a new one for the 73 Dart for front disc and rear drums.

I'll draw it out as you per your instructions and let you know if I have any questions.

Now's the best time to do this...my wife is often with me when out driving around.