70 challenger,
I have wilwood disc brakes front and rear, they are the 4 piston calipers.
I am currently using a factory 1.0" master cylinder (power brake, front disc/rear drum)

Car brakes good, but pedal feel is not ideal.
They are touchy, and the pedal travels quite far before getting a firm feel.

In my limited experience And research I would expect a better pedal feel by
Increasing to a larger master cylinder bore.
How drastic is the change going to be between 1/32, 1/16 & 1/8 going to be and which size would you reccommend?

Thanks for helping me improve the safety features of my ride!

Take a looksee here:

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

There is a link to a spreadsheet that will do the calculations for you.

The power assist will make braking easier by increasing the force you apply to the pedal but it will be linear.

Touchy how? as in barely touch the pedal & there's a significant amt of braking (as in more than normal)? you might check the booster nub freeplay as it might be too tight. EDIT OE booster? and was it (the booster) OK before the conversion?

I changed from a 1 1/32" MC to a 15/16" looking for more pressure and never could get a really firm pedal...went back to the 1 1/32". (Factory drum/disc)

I theorize that the extra pressure supplied by the smaller MC imparts more flex in the system components.

Yes oe booster. Touchy meaning there's lots of travel in the pedal with no feel or braking then near the back half of the travel the " braking" happens and they are touchy. Booster working fine. If I try to brake after disconnecting the vacuum line, it's very difficult to stop the car.

Thanks for the link to the spreadsheets Feets! Will try it out

could there be air in there

Sounds to me like air. Softness then all braking is not a characteristic associated with master cylinder bore sizing.

There is also no reason to try to test with the vacuum line disconnected either, that isn't going to help or diagnose anything.

Start with eliminating some basics that most overlook, are the bleeders on the calipers mounted higher than the pistons?, if not, it's possible you have air trapped in the system

did you adjust the end gap of the booster's pushrod?, if not your pedal travel is going to have a longer travel, and/or the piston in the master bore is likely not traveling it's full stroke displacing enough volume/pressure through out the system

Are you using a distribution block from a drum/drum system or a proportioning valve from a disc/drum system?


pic below is the booster to master pushrod adjustment needed to assure proper operation

Make sure to check the rotor runout on each corner. Excessive runout will cause a low pedal. DAMHIK.

what does using a drum/drum distribution block do to a front conversion, and if using one, do you remove anything internally to make it work ? or just bypass it altogether ? millions of years ago, i converted a b-body to front discs and did a couple things wrong. #1, i used a 1 1/8" truck master on a [10" drum if it matters] drum booster, but didn't check the booster pushrod depth in the master piston recess. #2, i used the drum distribution block to keep the "failure light" function. the results were not stopping very good with an extremely low pedal. i now realize #1 was a no-no for sure by using the drum single diaphragm booster coupled with not checking the pushrod clearance in the piston bore. what did using the drum distribution block do to screw up this setup even more than it already was ?

I had a 65 dart drum/drum and I went to 76 (2.75) OE front discs with ceramic pads and 10" (later BBP 7&1/4 so not sure the shoe width) rear drums with OE replacement shoes with an alum dual 1&1/8 MC & kept the OE splitter & it stopped flawlessly. it would lock the rears if I got on em hard (rarely did I do that) and pad wear was not noticeable and pedal effort was fine & definitely enough braking (but this was around town/not sure how it would do in a faster environment)

Fancy term for that is Pad Knock-Back.

Also caused by worn wheel bearings and the pad surface not being parallel to the rotor.

A drum/drum distribution block is nothing more than a "tee" or"branch" to distribute the fluid front to rear, and side to side for the front brakes, a disc/drum proportioning valve on the other hand will create a fluid bias front to rear, if your doing just a disc/drum conversion you need a proportioning valve, factory or aftermarket to create/maintain a braking bias, you could retain the drum/drum distribution block and add an adjustable PV for the rear drums on a disc/drum conversion, for a disc/disc conversion, you can keep the distribution drum/drum block if desired, you may need an adjustable PV valve for the rear line if the system wasn't designed with the proper components to have a proper fluid/pressure bias, but I would never use a FACTORY proportioning valve on a disc/disc conversion, no adjust-ability, and too much restriction to the rear line for a rear disc set up, a proper size bore and volume master cylinder is also a major concern for a disc/disc conversion, as well as a disc/drum conversion on a drum/drum vehicle

I installed 4 wheel Wilwood disk brakes on my Barracuda resto-mod (non power)and could not get a firm pedal after bleeding and bleeding. After looking closely, I noticed some deflection in the Al calipers when brakes were applied. I got out the dial indicator - and sure enough, the calipers deflected under pressure. This is a known problem with some of the Wilwood Al calipers.

Thanks for the suggestions and replies.

These callipers have four bleeders on each one and yes they are a pain in the ass to bleed all the air out, I feel I have done a good job getting most of the air out, however I did pull the master away from the booster and adjusted the pushrod out a few more turns, put it back together and noticed the pedal felt much better immediately. Will take for a test drive hopefully later today.
I am using the drum dist block and have a prop valve tee'd into the rear circuit.

May give the brakes another good bleed now that the I have a better stoke in the master cyl

Thanks everyone

When bleeding the calipers - take a soft faced hammer and gently knock on the caliper. This helps to dislodge the air bubbles.

you might put a block of wood under the pedal to it dont go past its normal travel range and it helps a helper control the stroke better (with you on the bleeder(s). bleeders only need to be open a slight bit to open them (dont want to pull air back in on the return stroke, made that mistake early on!). Holler how things turn out

Russell Speed Bleeders would be great for this one man operation.

Basically a bleeder with a spring-loaded check.

R.