Brakes: Disc brake conversion problems

[mbstude]

The facts:

1951 Commander.

Turner front disc brake conversion. Turner dual master cylinder bracket. Corvette style disc/drum master cylinder with remote reservoir.

No booster. No Hill Holder.

Installed the required 2# and 10# inline residual pressure valves.

I did NOT install a proportioning valve going to the rear.

Rear brake assemblies from a '63 Avanti, 11" drums. Brand new drums, wheel cylinders, shoes, etc. Adjusted per shop manual.

Master cylinder pedal freeplay adjusted to 1/4 inch of pedal travel before it engages the master cylinder piston.

All metal lines are new. All flex lines are new.

I've ran over a quart of Dot 3 fluid through the system and all bleeder screws shoot a clear stream of fluid with no air bubbles.

PROBLEM:

Brake pedal is "low".

No front braking power, period. It's as though the disc setup is nonexistent.

I can hear the rear brakes energizing when I press the brake pedal, and when driving the car around the yard, the rear brakes (slowly) stop the car.

Any ideas? Is this a result of not installing a rear proportioning valve? Air still somewhere in the system? Something else entirely?

[jts359]

Are the steel lines going to the correct side of the master cyl. ? And are you sure the master cylinder is returning to the stop position ? , I have done this swap without a proportioning valve and I had no problems , Ed

[mbstude]

Ed, everything is plumbed correctly. I have the correct pedal return spring in place and the pedal comes all the way up.

I talked to a mechanic-friend after making the post and his first suggestion was air still in the system somewhere. I bench bled the MC prior to installing it, but that doesn’t mean I got it just right.

[DEEPNHOCK]

Remember.. The residual check valves do nothing except stopping fluid from siphoning out of the lower master cylinder.
The proportioning valve just delays the onset of braking to the back wheels a little bit.
If you are confident the hydraulic portion of your system is OK, then you need to look at the mechanical side.
You say the m/c is a disc/drum. Are you sure? Double sure? A smaller piston will require a longer pedal stroke if the calipers and wheel cylinders are large.
Your pedal effort may go down, but the stroke to supply the volume may go up.A larger m/c piston will take less stroke, but your pedal effort will be higher to
get the needed pressure. A larger piston will give you the volume, but will have less pressure.
What brand/number master are you using? What is the pedal ratio (mechanical)?

[mbstude]

Jeff, this is the master cylinder. Went with a 1" bore per Jim's instructions. Pedal is stock Studebaker.

https://www.speedwaymotors.com/Unive...Bore,4316.html

[Dwain G.]

Where are the caliper bleeder screws located? They have to be at the top.

[bison]

Matt
i am going to ask some basic questions ( no offense intended ) as i just put did the Jim Turner dual master cylinder upgrade on my truck , although i did not install the front disk brakes . you said the lines were plumbed in correctly , so rear brake line goes to front inlet of master and front brake lines go to the back inlet on the master cylinder . The other thing i am thinking is because it is a dual master cylinder now , are both bowls full of fluid ( again , no offense ) another basic is the bench bleed of both chambers , again basics but thought i would just throw it out there .
like Jeff said , i dont think it has anything to do with the propositional valve . Could be a possible blockage in the front lines, hose kinked , wheel cylinder problem ?
When i did all my new lines i blew them all out with brake cleaner and compress air as i was afraid of something ending up in the lines.
Good luck and hope you get it figured out .

[Colgate Studebaker]

Matt, when we still lived in Wisconsin I had a '63 GT Hawk and it had drum brakes all around. I went through the entire brake system and I had a similar issue to yours. I replaced the master cylinder and all the rest of the usual stuff. It turned out that the check valve in the master needed to be removed so that the fluid volume would move properly. Check with Jim Turner to see if he knows anything helpful. Hope this helps, Bill.

[studegary]

Be sure to properly, manually adjust those old style rear brakes. I have come acoss many cars (Avantis) where they were incorrectly adjusted. This will cause an excess requirement of fluid to the rear. Of course that will not prevent the fronts from working. That is something else. Give another try at bleeding the calipers.

EDIT: In rereading, I do see that you say that the rears were adjusted per the shop manual.

[jts359]

The best way to make sure the master cyl is returning to its stop position is to slide the boot off and make sure the piston is all the way back on the piston retaining clip. Over the years I've had a few stick due to dirt and machine turnings , Ed

[bensherb]

Is your residual valve in the front brake line flowing the correct direction?

[mbstude]

Caliper screws are at the top. This MC is spec'd for disc/drum systems according to the info on the website, so it shouldn't have the check valve. I triple checked the residual valve direction, but I'll check again.

you said the lines were plumbed in correctly , so rear brake line goes to front inlet of master and front brake lines go to the back inlet on the master cylinder .

This may be my problem. I also received a message from someone else stating this same thing.

It has always been my understanding that on any dual MC, the port closest to the brake pedal goes to the front brakes, and the port furthest away goes to the rear brakes. This is how I plumbed the system on this car. I simply wasn't aware that it should be the other way around.

So it looks like I screwed up and plumbed it backwards.. I never would've guessed that, but this is also the first time I've ever dealt with this style of master cylinder.

Thanks everyone.. I appreciate the responses. If switching the lines around fixes problem, I'll be a happy camper.

I'll get the car back on the lift next weekend and update with the results.

[bison]

Matt
definitely look up our master cylinder application as it could be different than mine. the master cylinder i am using is a GM universal used on Corvettes with a 1" 1/8" bore and is used on my truck , your application could be different , but it just sounds like you might have it backwards .
good luck
the the way the # on my MC is MC2912H

[gjamesk]

Matt, FWIW I did the same conversion on my 52 and have the front brakes plumbed to the larger, rear reservoir, and vice versa for the rears. I believe this is correct and they're working fine. I put in a proportioning valve but have not needed to close it at all so may as well have left it out. Used DOT 5 and had a hard time getting them bled, banging on the calipers and rear cylinders with a rubber hammer helped free up some captured air.

[bensherb]

It has always been my understanding that on any dual MC, the port closest to the brake pedal goes to the front brakes, and the port furthest away goes to the rear brakes. This is how I plumbed the system on this car. I simply wasn't aware that it should be the other way around.

This is usually correct. To double check, the larger of the two reservoirs is the front disk brake one.

If it is reversed it will only make a difference , with a "single bore" cylinder, if the reservoir is too small to supply enough fluid. The brakes will still function properly, but if it's plumbed wrong it should be corrected.