O/T, but relavant, dual master cyl question

[buds truck]

A couple of weeks ago, I was towing my travel trailer with my 49 Plymouth and the rear brake line sprung a pinhole leak. it has a 72 Nova front susp and master cyl with an 8 inch Ford rear end. My question is... shouldnt I have still had some brakes since the front drum resevoir was still full. My brakes hit the floor and there was notheing there. Luckily I was a short distance away from home and was able to use the emergency brake to stop.

[JoelOkie]

Sounds like you didn't have brakes but on the rear axle to begin with. Joel

[buds truck]

when I put in the new brake line and bled it, I jacked up the side and checked. The front brakes hold as solid as the rears.

[48Dodger]

Ya...you should have had brakes, but....the front reservoir can usally be the rear brakes. I know you said the leak was from the rear...I'm wondering if you emptied the front brake resivor first. This would mean you had no front brakes and a very weak rear brakes....

48D

[buds truck]

front res was still full, the rear was empty. this is a 4 drum brake system. Is it possible that the guy who did the brakes plumbed it wrong???

[PatS....]

I have had brake failure over the years with dual pot equipped vehicles and in all cases I lost all the brakes.

The theory behind the dual pot system does not match my real world experience.

Not sure why it happened...in all cases they were not cars I had worked on at all!!

[Don Coatney]

Ya...you should have had brakes, but....the front reservoir can usally be the rear brakes. I know you said the leak was from the rear...I'm wondering if you emptied the front brake resivor first. This would mean you had no front brakes and a very weak rear brakes....

48D

I believe this depends on the orientation of the master cylinder. With swing pedals and a firerwall mounter master cylinder I believe the front chamber is for the rear brakes. This would also be the smaller chamber. However on an under floor mounted master cylinder the orientation is different. The large chamber faces the front of the car and this would be for the front brakes. Please correct me if I am wrong as I will soon be plumbing up my new dual master cylinder.

Small chamber rear brakes, large chamber front brakes, is this correct?

[Jerry Roberts]

Large chamber for the front discs , small chamber for the rear drums .

[JoelOkie]

For what it might be worth, on a stock Buick setup which I just removed from a car yesterday the smaller line from the large resevoir runs to the front of the manifold and supplies the front disk brakes. The rear brakes are drum, and are supplied by the bigger line. My 46 ford pickup, (which has Pontiac parts), has the same setup, large resevoir/small line to front disk, small resevoir/large line to rear drums. the large resevoir on both is at the front of the master cylinder. Sorry the pics are so blurry. Joel

[Don Coatney]

For what it might be worth, on a stock Buick setup which I just removed from a car yesterday the smaller line from the large resevoir runs to the front of the manifold and supplies the front disk brakes. The rear brakes are drum, and are supplied by the bigger line. Joel

Now I have another question for ECI. Should the line size be different front to rear? I notice on the Mustang master cylinder I am installing that the holes for the line size appear larger than what is on the stock master cylinder.

[oldmopar]

just taking a shot here but maybe the brake rod that goes to the dual mc may have not been long enough to push the fluid in the front reservoir.

[JoelOkie]

just taking a shot here but maybe the brake rod that goes to the dual mc may have not been long enough to push the fluid in the front reservoir.

Kind of what I was wondering...maybe a leak AND a master cylinder issue? Loss of brakes to both axles at the same time would be a master cylinder issue, right?

[JoelOkie]

Now I have another question for ECI. Should the line size be different front to rear? I notice on the Mustang master cylinder I am installing that the holes for the line size appear larger than what is on the stock master cylinder.

]

Don, the brake lines from the manifold (proportioning valve, or whatever), that run to the front wheels are smaller lines on my cars, (looks to be the same size as the small line that runs from the master). The line that comes out of the back of the manifold is a large line back to the flex line at the rear axle. These are just run of the mill stock applications in both cases using all the components from donor cars right down to the pedals, so I have no idea what the standard procedure is in add ons and re-fits. I'm curious to know what you find out regarding line sizing.

[Merle Coggins]

In the dual chamber/dual piston master cylinders that I've seen the front piston is actuated by the pressure generated by the back piston, plus the return spring. So, if you blow a line that's connected to the rear half you won't create enough pressure to activate the front piston. The front piston will only work if you have enough stroke to push the rear piston to it's end. Then you might get a little bit of movement on the front piston.

If it happens the other way around, the front section looses pressure, then the rear section will continue to operate normally.

Merle

[John-T-53]

Is the proportioning valve supposed to play a role if the front or rear circuit fails, such as close off that circuit to prevent further fluid loss? Any difference in pressure between both ends of the unit moves the valve in the middle, also triggering a warning light (if so equipped).