Projects built lots of cars, but never a 32 roadster...

Nope, it was residual fluid that leaked from the flex line fitting a few days ago and seeped inside the backing plate, down and out the bottom by the drum.
Back to the drawing board on the troubleshooting...uuugh,

 

If there was air in the system, I'd never get a hard pedal. So I'm looking more at the physics of the system...

The rear brakes are standard Ford 9" - have 3/4" rear wheel cylinders
The front Lincoln repros have a 1 1/8" wheel cylinder
A trusted drag racing builder buddy (who's knowledgeable brake system design) suggested that the 1" piston Corvette master cylinder I have is not adequate to pump the entire system up with one depression of the pedal. That may be the reason that the pedal gets hard (and stays hard without bottoming out as long as you keep it depressed) after you pump it up a few times.
A 1" bore at 1" of travel moves 20% less volume than a 1 1/8" bore at the same travel.
Jason at Pete and Jake's is sending me a 1 1/8" bore MC - hoping that solves the problem.

 

I'm running GM disc brakes on the front and 11" Ford drums in the rear, a typical set-up, and am using a 15/16" bore master cylinder. The system works great. 1 1/8" bore seems huge. Maybe someone with your set-up will chime in.
The residual valves should hold pressure in the system to prevent the problem your buddy mentions. The bigger the m/c the more pressure required to slow down.

 

Good reading:
https://www.motor.com/magazine-summary/performance-perspectives-master-cylinder-selection/
Also, is your pushrod allowing the m/c piston to fully retract. This opens the small hole in the reservoir that refills the piston bore, very important.

 

I appreciate your input - please keep throwing ideas at me...
There's a lot of info out there on "upgrading to discs" or master cylinder sizing for disc brakes, but none of designing/sizing for 4 wheel drums.
Remember, there are return springs on shoes with drum brakes (that pull the shoes back when pressure is released) versus the fluid volume staying in the caliper with discs. The residual valves only hold back 10 psi (you can't really compress a liquid). With a 5:1 pedal as I have and a 1" diameter MC, that's around 371 psi on the wheel cylinder piston face.
I believe I have a volume problem, not a pressure issue.
My push-rod/pedal geometry fully allows clean insertion and retraction of the push-rod/piston.

 

So, exactly how much pedal free play do you have at the pedal?

 

My experience with four wheel drum brakes is that they work well if the wheel cylinders are similar diameter front and back. Then, a similar diameter master cylinder to wheel cylinders. This has produced good balance, from to back.

 

I hear you on the volume issue but on the pressure side, 5:1 pedal ratio is at the skinny end for a manual system ( I'm not recalling a booster here); 371psi or thereabouts seems rather low to provide decent braking, well certainly for discs but I'm presuming drums will require similar levels, and swapping the 1" for a 1 1/8" master will lower that pressure further, all things being a equal.

There shouldn't be so much slack to be taken up before the shoes meet the drums such that volume becomes an issue. Are the brakes adjusted properly before bleeding? Or are these brakes the type that should be fully unadjusted (can't immediately think of a better way of saying that right now!!) before bleeding to be adjusted up subsequently?

Chris

 

I hear you on all points, maybe this master is just garbage? I don't know.
At least I know I don't have any leaks, so I went ahead and put the exhaust and engine in tonight.

 

Please define "free play"

 

Freeplay: If you touch the pedal pad with your finger, it should move a bit, BEFORE the rod starts pushing the master cylinder piston.

If not, adjust the rod a little shorter so you have some Freeplay at the pedal pad. This allows the piston to withdraw all the way when you let off the brake pedal.

 

The weight of the pedal keeps the push rod seated in the piston - there's no play. If you pull up on the pedal, you will pull the push rod out of the piston (when I put the body on, the toeboard will make that impossible to happen.

 

Can’t check the free play till the floor is on and the return spring is holding it up to the rubber bumper on the bottom of the floor.

 

Important.

 

All correct. The free play should be just a wiggle of the pushrod. About the equivalent of a red ch

 

Scott,
If your brake shoes are reasonably arced to the drums you could adjust the "star adjusters" at each wheel to lock up all wheels, simulating a brake stop. The pedal shouldn't have to add a large amount of fluid in this situation. If the pedal still travels a long way there is another problem.

 

Such as?

 

Air still somewhere in the system?? Although you seem to have that solved with the pressure bleed. If the pedal pumps up then holds pressure I would think you are compressing air, no? Just guessing at this point.

 

Yeah, it's either air (doubtful)...a bad chinese master cylinder (ordered a new 1" one from tuffstuff)...or the wrong piston size (which the 1 1/8" MC will be my last resort). While I wait for the MC to arrive, I'm going to start wiring it.

 

Scott,
That's going to be sweet when you're finished! Especially that windshield! heh-heh!

 

POS repro Corvette master cylinder #2. This one, a Tuff Stuff unit from Summit racing. I'm Getting tired of painting master cylinders. On the positive side, at least it leaked on the bench (before installing it in the car).

 

Fingers crossed…

 

You mean for the next/third one? I'm ordering it tomorrow.

 

How annoying. Third time lucky.

Just to be certain, when you're bleeding the master, whether on or off the car, you have a slight inclination up at the pushrod end, down at the opposite end, right?

Chris

 

Yup... Didn't matter with this one anyhow, it's leaking like a sieve from the rear of the piston

 

Called Summit Racing, third MC is on the way.
I told the guy at Tuff Stuff that I'm getting tired of painting these (Using good $EM primer and Ea$twood cast iron paint) - he said that might be an issue for returning, not to tell Summit...unbelievable.

 

One thing I love about SBC ram horn manifolds is how neatly & easy the plug wires arrange...The only thing I have on there is 2 P clamps on each side, attached with existing holes in the block - no looms no dividers, no zip ties, nothing

 

Glad to see that you are still making progress on your car…where are you at with your breaks? What length ARP bolts did you use on your exhaust manifolds? I really like the look of the routing of your plug wires.

 

Thanks - I mocked it up and then measured the depth of the hole in the head through the exhaust manifold with a gasket on it...then bought 2 different lengths of bolts from ARP and then cut them down to the correct length on a lathe.

 

This is the first time I've done sound deadening material on a car with a removable floorboard, firewall and transmission cover - it's great because it's easier than crawling up under the dashboard - but I find it amazing how much weight it adds to the pieces