At this point I'm pretty sure I was sold too small of an MC for my disc brake swap on my 67 Coronet. It's 1 1/32", and with 2.75" piston Wilwood GM Metric calipers in the front and 15/16" wheel cylinders in the rear, my pedal is long and mushy and I have no ability to lock the front brakes (manual). I had a harder pedal and more power when I was on all-drum.

If I clamp the rear flex line, a good 80% of my pedal travel is just getting the front brakes from rest to loaded against the discs (this is not a bleeding issue, I can't pump up from this). I do have stopping power, but effort vs output really ramps up beyond moderate braking. Nothing happens with the pedal until it's almost parallel with the top of the gas pedal (I have moved the pedal by hand and listened in my garage, that is just the stroke required to take up the front brakes).

I have been trying to figure out what the best size is to step up to. I have a universal/GM-style two-hole master so I can pick from a lot of stuff. How do you determine this?

I can use 1 1/8 from a 70s D-series, or 1 1/4 for an 80s K1500 with regular OE Metric calipers and huge rear drums, etc. I don't think 1 1/16 would be a large enough step up considering how long and squashy my pedal is now.

I thought you needed a proportioning valve on the rear brakes when swapping from drum to disc front brakes. But I'm not positive.

What's the pedal ratio?

I have a prop valve, again I have clamped the rear flex line. This is not a rear brake issue.

The pedal ratio is whatever a stock all-drum car is. I have one hole on my pedal arm and it's about an inch and a half below the pivot.

This is an MC size issue. I've diagnosed and checked everything else away, believe me.

Realizing your concern on excessive pedal travel, going to a larger piston on MC will certainly decrease it, however at the expense of lessened clamping pressure on front disc,,,which you indicate is not sufficient with your manual brakes to lock front wheels.

Are you certain that your new MC was properly bench bled, that brake lines are truly purged of all air,,,including bleeding at both valves on each caliper,,,that if using dot 5 brake fluid that it did not get shaken up before use which will cause millions of microscopic bubbles, that there is no flex in calipers when pedal is depressed hard.

Are you bleeding the old fashioned 2 person method,,,I have an expensive Mity vac air compressor vacuum system,,let's just say I have become a confirmed skeptic. Oft times the old ways are better.


My suspicion is that a 15/16th MC would be the way to for the front disc brake pressure you require to lock up,,,,your long pedal travel is a problem that lies elsewhere.

Just throwing some thoughts out there.

Yes it is bled to death. The MC has been bench bled and I've done a few crack-the-line bleeds on it too. I'm using DOT 4. I have vacuum bled, pressure bled, gravity bled. There is no air, I can't pump this slack up. It takes that much pedal stroke just to go between rest and loaded.

so 2 things.
I agree with the others that smaller diameter will give you more braking force and you should probably pick up the manual master for the 70's corvette. it is the smallest GM one that I could find.

2nd, the pedal take up you describe does not sound right.
either you have some play in the front discs that you don't see, causing the pistons to push back, or there is something drawing the pistons back in.

but please explain how you know it is only the take up to get the piston to the disc?

I'm thinking it may be a bad master cylinder or defective flex lines? If all the air is bleed out, you should be able to lock the front disks.

I agree that something is wrong and its not the MC size.

In my 67 Coronet I have 76 Volare 2.75" calipers with 10 x 2 1/2 rear drums (don't know the wheel cylinder size). These are manual brakes.

I run the Doc Diff 15/16" master cylinder and my pedal is just fine. Does not have excessive pedal travel.

I have moved the pedal by hand from rest to its take up point, and between those points I can hear the brakes move. When I have the calipers flipped up and held on with a C-clamp, I can see the clamp screw end move slightly in sync with the sound as I have to start pressing harder.

I use Dr Diff's as well

I just noticed I have sediment in the front reservoir...I have new hardlines and flex hoses. This thing must be busted inside. But my pedal doesn't sink at all?

I still think you have something causing the pistons to be pushed back in.

if you compare any other car with front disc, you shouldn't get any noticable movement of the calipers when you start to press the pedal down.

There is some minor retraction due to the o ring around the piston, but that is to take the drag off.

if you have enough movement to hear it something is wrong.

One of the old drag racer tricks was to loosen the nut holding the hub on so the disc rotor had some wobble and it would push the pistons back in as the car rolled giving less drag on the track.
hard on the bearings, but it seemed like the car would go faster.

I would ask you to see what kind of (I think the term is) run out your rotors have as you turn them.
see if they aren't pushing the pistons back in.

Take a look here:

https://board.moparts.org/ubbthreads/ubbt...ed-to-know.html

PIE are round.

Are you sure the booster pushrod to master clearance is right? Sounds to me like excess clearance.

What does the pedal feel like if you clamp off the front hoses?

As said if power I would check the round nub clearance & if OK then I would clamp/or block off all 4 corners & see if the pedal is high an tight & if good then the prob is downstream & I would proceed to examining the corners by releasing LF then recheck then RF then recheck then rear. we gotta pin this down

i'm just throwing this out there, but something in the back of my mind [it's a loong way back there to be sure...] keeps telling me gm masters are plumbed opposite of mopars, the rear port is for the rear brakes, while the front port is for the fronts. you don't by chance have the master ports reversed ?