master cylinder sizing.

[keithq69]

I have a 69 Camaro that I bought recently.
The front brakes are 95 ZR1, HD 13" rotors, PBR calipers.
The rear is a Ford 9" with an older style cast caliper.
The car has the DSE master and booster setup.
The pedal has always felt a bit soft to me, there now seems to be something wrong with the master and it needs to be replaced.
I'm considering a new master with a larger bore. I know this will reduce pedal travel and increase effort.
I think the system was over assisted anyways and the pedal travel felt a bit too long.
The existing master is a 1" bore S10 as far as DSE said.
The 95 ZR1 used a 1.25" master but the fittings are on the wrong side for my application. I know I could change them but would like something that bolts in.
Any suggestions?

[jpgolf14]

I have a 69 Camaro that I bought recently.
The front brakes are 95 ZR1, 13" rotors, PBR calipers.
The rear is a Ford 9" with an older style cast caliper.
The car has the DSE master and booster setup.
The pedal has always felt a bit soft to me, there now seems to be something wrong with it and it needs to be replaced.
I'm considering a new master with a larger bore. I know this will reduce pedal travel and increase effort.
I think the system was over assured anyways and the pedal travel felt a bit too long.
The existing master is a 1" bore S10 as far as DSE said.
The 95 ZR1 used a 1.25" master but the fittings are in the wrong side for my application. I know I could change them but would like something that bolts in.
Any suggestions?

That is interesting. The C4 HD brakes are some of the shortest travel / highest effort brakes out there. Granted the 1.0" master is quite small for power brakes and will kill quite a bit of those characteristics.

The 1.25" master is 56% more area than the 1.0". That will really change how the brakes feel.

Try the mid 90's B-body HD master. Delco part number 18M641. Should be a 1.25" bore.

John

[keithq69]

Thanks John.
That's the One I was considering but the fittings are on the passenger side of the master not the drivers.

Am I on the right track?
Maybe I should stick with the 1" bore.

[jpgolf14]

Thanks John.
That's the One I was considering but the fittings are on the passenger side of the master not the drivers.

Am I on the right track?
Maybe I should stick with the 1" bore.

Keith,

Are you sure about that? All my research suggests that master should be a drivers side exit. Not sure if the fittings are the correct style for your current lines though.

Yes, I think a 1 1/8" - 1 1/4" master is more suitable. Personally, I like a short firm pedal. Personal opinion.

[Apogee]

I thought the 1992-1996 C4 Corvette used a .94" bore MC (Raybestos #MC390255)...where are you coming up with 1.25"?

Since you didn't specify which PBR or "older cast iron" calipers you're using, it's a bit tough to be specific, however with a dual-9" DSE booster I would probably recommend a 1" to 1-1/16" bore MC as a starting point. If you wanted a slightly higher, firmer pedal, then you could certainly go 1-1/8", but any larger than that for the assumed caliper piston areas would seem a bit excessive to me.

Tobin
KORE3

[jpgolf14]

I thought the 1992-1996 C4 Corvette used a .94" bore MC (Raybestos #MC390255)...where are you coming up with 1.25"?

Since you didn't specify which PBR or "older cast iron" calipers you're using, it's a bit tough to be specific, however with a dual-9" DSE booster I would probably recommend a 1" to 1-1/16" bore MC as a starting point. If you wanted a slightly higher, firmer pedal, then you could certainly go 1-1/8", but any larger than that for the assumed caliper piston areas would seem a bit excessive to me.

Tobin
KORE3

I was assuming he is using the C4 HD calipers with 1.50" pistons.

Tobin, do you have a good idea of a typical gain in a vacuum booster? This information is very hard to find. The best I found was 4:1 gain from an old brake book I have around.

John

[Antti66ht]

Have you considered wilwood master 1, 1/8.

I have 4 piston calipers fore and back, and 1" felt very soft, but a 1 1/8 felt just right. I will see if it is big enough for RS4 eight pot fronts... I believe 1 1/8 is biggest wilwood offers. The connections are on both sides.

[keithq69]

Hey Tobin, thanks for the reply, I know you deal with this stuff all the time.
I included in the first post that they are 95 ZR1 13" HD PBR calipers, I'm not sure of the piston size.

I got the master cylinder sizing from this site http://lukeskaff.com/?page_id=333

Just went back and read it again, the 1.25" is for the B body. it said HD brakes and I assumed it was the Vette stuff since it was referencing the C4 HD stuff above.

I just don't want to have the same issue with poor pedal feel again if i use the 1" master.

What would the part number be for the 1 1/16" master? Or what vehicle would it be found on?

Thanks

[Apogee]

I was assuming he is using the C4 HD calipers with 1.50" pistons.

Tobin, do you have a good idea of a typical gain in a vacuum booster? This information is very hard to find. The best I found was 4:1 gain from an old brake book I have around.

John

Gain will vary depending on the booster size and configuration, as well as the vacuum levels supplied to it and is not particularly easy to calculate directly. The easier method to calculate gain for a specific application is to measure pedal input force versus output pressure at the master cylinder or caliper, both with and without the engine running. This should give you a gain multiplier for the system as it sits so long as your input forces are the same.

Tobin
KORE3

[Apogee]

...I got the master cylinder sizing from this site http://lukeskaff.com/?page_id=333...I just don't want to have the same issue with poor pedal feel again if i use the 1" master.

What would the part number be for the 1 1/16" master? Or what vehicle would it be found on?

Thanks

There are several OE applications that used 1-1/16" bore MC's including many Mopar applications from the mid-80's to mid-90's IIRC. Wilwood sells what is essentially the same thing, 260-4893 (http://www.wilwood.com/MasterCylinders/MasterCylinderProd.aspx?itemno=260-4893). Strange Engineering sells a similar unit with a 1-1/32" bore, also aluminum with a black reservoir.

Another unit recently brought to my attention by Carl Cassanova is the 2006-2008 Trailblazer unit with 1-1/16" bore, but I don't know anything more than that.

Tobin
KORE3

[keithq69]

I'll look into the 1-1/16" bore.
I still think the added bore will help with pedal feel..

[jpgolf14]

I'll look into the 1-1/16" bore.
I still think the added bore will help with pedal feel..

The 1-1/16" bore has a 13% larger area than 1.0". I would imagine the 13% will be just noticeable, but hardly a dramatic difference.

[keithq69]

I've had a look online and from what I can tell the bore on the Trailblazer is 11/16' not 1-1/16".
I'm going to see if I can get a hold of one of the techs to get a spec on it. I've been trying to find a metric size but haven't been able to.

Keith

[keithq69]

Carl was right, the size is 1-1/16", I just confirmed it with Raybestos tech line.
I guess the other sites forgot to include a dash or space between the ones.
I've ordered part# MC390983 so I will install it tomorrow, head to the track on Friday and see how it works.

Keith

[keithq69]

I tried out the 1-1/16" master at the track yesterday.
The install went well, the reservoir was a bit taller than the S10 master but it still fit under the hood no problem.
I think I still have a bit of air in the system but this master seems to have reduced a bit of the assist.
I always thought the other master felt over assisted.
I'm going to look into ABS for next season. I found that at the end of the first high speed straight I had trouble threshold breaking without locking the tires a bit.
I think I need to dial a little more rear brake into the proportioning valve.

Keith.