Master Cylinder bore size for Wildwood Dynalite PWR disc/drum

[68 Sport Satellite]

Hi all,
Early on in my car build I did a lot of research on brake MC bore sizes and went with a Raybestos blue 15/16". Stopping is adequate, but pedal feel is not. MC and brakes have been bled 3 times by pro shop. I'm finally finished with most of my build and want to get better brake feel. Here are the specifics:

68 satellite, power brakes, front disc/rear drum. 8" dual diaphragm booster, 4 piston wildwood dynalite calipers up front, rear drums with reduced bore wheel cylinders for brake proportioning.

Current MC is 15/16" raybestos blue. Car stops, but pedal travel goes too low before full bite. Overall pedal feel is ok, but cannot stop as quick as I want. Brakes and MC have been bled by shop multiple times.

I'm going to try a larger MC to see if the fluid can fill my caliper pistons more readily, but I still want a nice feeling pedal that I can modulate.

For those who have been down this road, what was the result of you trying a larger MC bore size for power disc/drum? With power disc/rear drum and the wildwood dynalite's, I'm not sure which of the following would be best:
1.00"
1-1/32"
1-1/16"
1-1/8"

Seems like such a 1/32" or 1/16" of an inch can make a big difference. I don't want a rock hard pedal, but I want the brakes to grab before the pedal is 4/5 of the way to the floor.

I'm also not sure what is available from which makers. Thinking of sticking with Raybestos, but I'm open to suggestions (except I am not considering hydro-boost). I do know about some of the folks out there like Dr Diff, but I want to hear from my mopar brothers and sisters if you were able to correct the same issue I'm having.

Thanks in advance!

 

[Jerry Hall]

Bleeding your brakes is one thing and I'm sure you have probably addressed that enough. Petal feel comes from the rear brakes. Have you made sure your rear brakes have sufficient drag, because until they have pressure on the drums, the petal is going to continue to fall. Personally I would not have reduced the bore size on the rear wheel cylinders, but I'm not the expert to ask on that. Remember this is just my opinion, there will be many more hopefully more experienced than me.

 

[steve340]

Have you checked the clearance between the booster push rod and the master cylinder piston?
That is the first thing I would check. I close them up to about 0.008" clearance.
Some will say it is too close but I have done this many times and it works and you get a nice high pedal.

 

[68 Sport Satellite]

Have you checked the clearance between the booster push rod and the master cylinder piston?
That is the first thing I would check. I close them up to about 0.008" clearance.
Some will say it is too close but I have done this many times and it works and you get a nice high pedal.

Thanks for that. What MC bore size are you using?

I set that clearance up with a brake specialist about 4 years ago. Can't recall the gap, but I recall it being fairly close by eyeball (maybe not .008" close). Any way to check this without unbolting the MC from the vacuum booster?

Still looking for feedback from real world use on pedal feel/travel for a power disc/drum car, stock pedal ratio linkage, for differences between these bore sizes:
1"
1-1/32"
1-1/16"
1-1/8"

Thanks!

 

[Fran Blacker]

From Mopar Action disc swap, he's talking about manual brakes. Note smaller bore for manual brakes so a larger bore for PB. Not a lot help.

Read last sentence in first picture and all of second pic.

 

[68 Sport Satellite]

Bleeding your brakes is one thing and I'm sure you have probably addressed that enough. Petal feel comes from the rear brakes. Have you made sure your rear brakes have sufficient drag, because until they have pressure on the drums, the petal is going to continue to fall. Personally I would not have reduced the bore size on the rear wheel cylinders, but I'm not the expert to ask on that. Remember this is just my opinion, there will be many more hopefully more experienced than me.

Actually, I am incorrect - I actually reduced the rear wheel cylinder pressure by going with a different size cyinder to prevent rear wheel lockup by following the advice in Rick Ehrenberg's brake proportioning article. These may have been larger than stock (or smaller? less pressure for sure though). Sorry for the confusion. I actually ordered the new rear cylinders from Rick personally.

 

[Jerry Hall]

Rick's article had something to do with the residual valve not being in some master cylinders and if it wasn't in the master cylinder you have then you use his rear wheel cylinders. This could be your problem (a mismatch of parts). Something to look at anyway.

 

[steve340]

I have a Challenger and a GTX both have the Hemi Bendix booster and 1 and 1/8" Ramman Hemi masters. Both cars have wonderful brakes and nice pedal.
There is no way I know off to check the clearance with the master cylinder installed.
I modify the stock Mopar pedal linkage to raise the brake pedal lever. I have done this to both my cars and a friends 69 Super Bee and the mod makes the brakes 8-10% better.
I really don't know why but it is a fact confirmed by my buddy when we did his.
You would have used smaller diameter wheel cylinders to reduce their effect.
If your Wilwoods have large diameter pistons you may have to go up in master cylinder diameter to displace enough fluid - however this change will lower the line pressure to the disc calipers.
I would recommend you purchase a brake pressure testing gauge kit and plug in to your front brakes and that will tell you what is going on.
If you cannot generate at least 1250 psi in the caliper the brakes will most likely be no good.

 

[68 Sport Satellite]

I have a Challenger and a GTX both have the Hemi Bendix booster and 1 and 1/8" Ramman Hemi masters. Both cars have wonderful brakes and nice pedal.
There is no way I know off to check the clearance with the master cylinder installed.
I modify the stock Mopar pedal linkage to raise the brake pedal lever. I have done this to both my cars and a friends 69 Super Bee and the mod makes the brakes 8-10% better.
I really don't know why but it is a fact confirmed by my buddy when we did his.
You would have used smaller diameter wheel cylinders to reduce their effect.
If your Wilwoods have large diameter pistons you may have to go up in master cylinder diameter to displace enough fluid - however this change will lower the line pressure to the disc calipers.
I would recommend you purchase a brake pressure testing gauge kit and plug in to your front brakes and that will tell you what is going on.
If you cannot generate at least 1250 psi in the caliper the brakes will most likely be no good.

Thanks for all of this info Steve. Very thorough and should help my investigation. Yes, I had heard that going up in MC bore size could displace more fluid to fill the Wilwood calipers.

It sounds like you've had good results with a 1-1/8" MC bore size. Thanks for that, good to know.
Now I just need to decide if I want to go up to that size or perhaps try a 1-1/32" or 1-1/16".
For reference in feel and engagement, my new 2019 honda civic has amazing brakes in every way. The pedal takes little effort to push but has nice pedal feel that is not too soft when fully engaged, it engages at about 1/4 of the way from the top of travel, full braking pedal engagement position is about 1/3 of the way off the floor, is easy to modulate and stops on a dime. I feel like if I can get this MC dialed in things will be much better (I know it's a totally different car and setup, but it's my reference point).

How would you describe your brakes that feel good with the 1-1/8" bore MC?
Effort to push =
Engagement point from top of travel =
Approximate pedal travel stopping point =
Easy modulation =
Pedal feel when fully engaged? (nice firm pedal or a bit soft?) =
Bites hard and stops quickly if needed =

 

[steve340]

Effort to push with those big Bendix boosters = easy
Pedal effort = easy and the cars stop pronto
Engagement = right at the top say 1/2 inch of travel at round town speed this is all you need do.
Well modulated no slamming on.
Firm pedal.

 

[themechanic]

Wilwood recommend I buy their 1 1/8" MC for my 4 wheel power disc system. They needed to know the piston bore size of the front and rear calipers to determine it. Pedal if firm with light pressure and begins stopping about 1/4" down.

 

[68 Sport Satellite]

That’s what I needed to know. Thanks guys. I think I will try the 1-1/8” and if it’s off a little I can back off to a 1-1/32”.

 

[Kern Dog]

Bleeding your brakes is one thing and I'm sure you have probably addressed that enough. Pedal feel comes from the rear brakes.

Not always.
I usually follow the standard rule of going to the FURTHEST away from the master cylinder to bleed first, the right rear, then left rear...A few months back I was bleeding out the brakes in "Jigsaw" and the rears just weren't getting much volume. The Wife suggested that I try the fronts a few times since the calipers have more room inside to hide air bubbles. She has listened to my stories for years and has caught on to some of the tricks.
She was right. After 2 passes each at the calipers, the fluid volume to the rear increased a LOT. My guess was that the fronts had so much air in them, the pedal feel was NOT going to improve until most of the air had been forced out.

 

[68 Sport Satellite]

Not always.
I usually follow the standard rule of going to the FURTHEST away from the master cylinder to bleed first, the right rear, then left rear...A few months back I was bleeding out the brakes in "Jigsaw" and the rears just weren't getting much volume. The Wife suggested that I try the fronts a few times since the calipers have more room inside to hide air bubbles. She has listened to my stories for years and has caught on to some of the tricks.
She was right. After 2 passes each at the calipers, the fluid volume to the rear increased a LOT. My guess was that the fronts had so much air in them, the pedal feel was NOT going to improve until most of the air had been forced out.

That's great info Greg! I will keep this in mind when i swap master cylinders and go down this road again. So you bled the fronts first twice, then the rears and all was good? (and of course bench bleed the MC first prior to install)
And what bore size is your MC may I ask?

I like that your wife listened enough to make this astute observation while you were scratching your head. That must have made you smile with pride when it worked out! Give that woman an ice cold beer!

 

[Kern Dog]

What I did was try to bleed the right rear first. I was getting a really weak flow from the bleeder after 3 tries. I tried the left side, same thing: really weak , just dribbling out like an 89 year old guy with prostate trouble. I went back to the right side when the Wife suggested to try the front. I went to the right front, pump/pump/pump then hold....a spastic mix of dirty fluid and air. I don't recall if I went 2 or 3 times at each side but the pedal firmed up a LOT. I went to the rear again and was able to get a strong stream, finally. It still had air mixed in but the stronger pressure helped evacuate the air much better.

 

[68 Sport Satellite]

I swapped from my 15/16” master cylinder with my power front disc rear drum to 1-1/8” master and the pedal firmed up and raised up just enough so all is better. Not too firm at all and now not all the way to the floor. A 1-1/32” may be a good middle ground but for now I’m happy. My mechanic said that changing out my aftermarket dual diaphragm 8” booster could give even better braking but for now I’m content.

 

[Mike67]

I could be wrong here but on Wilwoods site I believe I read that MC sizing is based mainly on your pedal ratio.
https://www.wilwood.com/Tech/TechQA