i have a 58 F100, on the way back from Primer Nats. car show the stock single jar started leaking. so i decided to upgrade to a dual master cyl. i bought one for a 69 Mustang w/ manual brakes and a for a big block. i put it on and for some reason it just slows me down if had to do panic stop i would keep going. if i pump them twice it feels better. i've bleed the brakes with a mighty-vac that work some then i did it old school with a jar and fluid. it's gotten a little better but it just slows me down. i checked for leaks and everything is tight.


what else can it be? maybe they gave a master cyl for pwr brakes? my leaky stock master cyl stopped me better. i might just rebuild the stock one and put it back on.


i followed this tech article....

https://www.ford-trucks.com/article/...ough_1964.html

 

What kind of brakes did the mustang have disc front and drum rear or drum front and rear? If it had disk front brakes the setup may not work well for you.

 

that i'm not sure.... i told the parts guy to give me a 69 mustang master for a manual drum brake set up with a big block. so i just figured he handed me the right one. i call up tomorrow and ask them to run the part #.

every so often i'll get a firm pedal the way it should be then it's soft. i've bleed the brakes like 6 times already. the pedal still feels the same way.

 

I didn't read the article did it cover bench bleeding the master cylinder?

 

If the two sections are the same size, you have the all drum set up. The one for disc/drum has two different sizes. If you have the right one it sounds to me that the master cylinder is either no good or has air in it. Did you bleed the master cylinder before the brakes? Usually they supply two plastic fittings and two hoses to do it. I'd stick with it, there is no reason it shouldn't work and the dual set up is much safer.

 

You could have a master cylinder for disc/drum setup. If you do it may have the wrong residual pressure valves. Drum brakes require a 10psi valve to keep the break lining springs from pushing all the fluid back to the master cylinder from the wheel cylinders. Disk brakes use a 2psi valve to do the same. This may be why the brakes get better if you pump them. According to the MP Brakes (www.mpbrakes.com)catalog disc brake master cylinders are larger than drum brakes and don't have built in residual pressure valves.
Maybe you could try this, Pump the brakes up so they are hard, have someone watch the master cylinder, when you take your foot off the pedal does the master cylinder fill back up.

 

I read the article and it was not clear on the master cylinder - it looks to me like you do have a front disk brake cylinder. If the front and rear brake fluid chambers are different sizes, it is the wrong one.

I am no expert but I would guess that 68 to 70 big block mustangs came with disk brakes, which is a conversion he is touting in the article.

Like Earl says, the fun never stops.

I am not sure there is a master cylinder that fits your need without a lot of modifications - if you find one please post the details as I also want to make this change!

 

I think you have residual air in the system. Probably do need to bench bleed the MC, then rebleed the system. I prefer to bleed a new setup backwards from the recommended method, starting with the wheel closest to the MC and working to the one furthest away. That way you are evacuating the majority of the air thru the shortest line, requiring the least amount of refills of the MC. Never really made sense to me to push all the air thru the longest line! Be sure you don't empty the MC between bleeds and reintroduce air. Note: it may take several bleeds to remove all the air from the longest lines especially if there is no residual valve the air will want to migrate back towards the MC. Did you adjust the brakes before starting the bleeding? If not, do so. Using a disk/drum or disk/disk MC on a drum/drum system should not pose any issues. The only difference is the size of the rez, disks require more volume, unless there is a built in proportionimg valve in the disk/drum MC. The MC really doesn't know what kind of brakes are on the other end.

 

Ax, sorry to be so slow this morning, but I don't see this. I agree the MC does not care what is on the other end of the hose - a wheel cylinder or a disk caliper would look the same to it.

What bothers me is the volume of fluid each side of the MC delivers per stroke.

On a drum/drum system both the front and the rear need basically the same volume of fluid to cause the wheel cylinders to expand and move the brake shoes the same distance - if the front needs 2 cubic inches of fluid, the rear would also need about this amount.

Depressing the brake pedal say 4" would cause both the front and the rear to pump 2 cu in into their respective systems, applying the two front and the two rear brakes the same amount.

On a disk/drum system, the rears still need the same 2 cu in of fluid, but the front needs say 4 cu in - so the master cylinder is built such that depressing the brake pedal 4" would cause the rear to pump the same 2 cu in of fluid into the rear but would pump 4 cubic inches of fluid into the front system.

The engineers who designed the system thought these volumes would give both the drum wheel cylinders (2 cubic inches) and the disk caliper (4 cubic inches) the fluid needed to apply the brakes.

It is even more complicated as they want the front to do most of the braking - somewhere around 55/60 % front to 40/45 % rear. But the point remains the same - the front system is designed to pump more fluid than the rear is for the same stroke of the brake pedal.

Now we come along and put this master cylinder on a drum/drum system. The front drums are receiving twice the fluid the rears do for the same stroke of the brake pedal. What happens is that the fronts apply quicker, and harder than the rears. I expect the front wheels would be locked up and the rears just starting to touch the drums.

In effect, the fronts would be doing all the braking and the rears coasting. It seems to me that this sort of mismatch would cause you to lose up to half your braking ability.

It is early, and my brain lost a few more cells last night, so I admit there is a lot of things I don't know, and lots of things I do know but have forgotten. So please tell me where I am wrong in thinking a disk/drum MC will not work in a drum/drum system?

 

WillyB -

Your explanation seems very clear-headed to me and seems to be the correct diagnosis of the problem. If that's what your brain cells do when compromised, it must be scary when you're on your game.

I keep losing brain cells too. I just recycle the working ones.

BTW, if you can remember that you do know stuff you forgot, you can rule out oldtimers disease.

 

okay i know for sure it's for drum brakes. both res. are the same size. the front of the master is going to the front and the rear to the rear. i did bench bleed the brakes but i just remember that when i was doing it i was called away to do something. so i might still need to bench bleed it some more. i will do that and rebleed the brakes with the mighty-vac and old school. the pushrod that came with the new master was exactly the same length of the old one.


thanks for all the help guys. i'll give you guys an update soon.