Bee Jay

First, I want to thank Noonie for telling me about this on my hood mod thread:
Then, I'd like to thank my good friend Gill for giving this to me as a Christmas gift.

But, it didn't fit because the sleeve is 1.75" in diameter, and this hole in the Hydroboost is 1.6" in diameter.

So I did some research and found out that the M2904 is build for hydroboost and will fit. The only difference is the bore size is slightly larger, from 1.125" up to 1.250". I'm not sure what effect this will have on braking or brake feel, but I have the M2904 on order with NAPA and should be here tomorow.
BTW, the stock Master Cylinder weighs 10lbs and the M2903 only weights 3 lbs. So I'll lose 7 lbs.
I also took this opportunity to turn my hydroboost upside down like everyone else does.

Stay tuned.
Bee Jay

TheSkunkWorks

The larger MC bore will increase fluid displacement for a given pedal stroke, and increase pedal effort.



edit - For those who heel-and-toe, changing MC bore diameter will likely require readjusting the relationship between the brake and throttle pedals.


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Gordonm

Keep us posted on this. Ive always wanted to replace that big lump on the end of the hydroboost.

noonie

Glad your working on this, you'll like it.

An 1-1/4" bore master with a stock vacuum booster gives a C3 almost perfect feeling brakes. Less mushy and higher pedal.
I ended up keeping a cast iron 1-1/4" master just for the closer oem looks. I would at least do this mod on any C3 I owned.

Just wondering which hydraboost you have. I've used the m2903 with the hydro as one piece from a upick yard. Didn't realize there were so many different pilots.
Hydraboost with the 1-1/8" is almost too touchy, but you can get used to it.

Double check to make sure what the bore is on the m2904, I just looked it up and it said 1-1/8".

I see if I can find any info I hung onto.

nate99

So, will that MC work on a stock vacuum booster (mine is an '81 if it matters).

groovyjay

Just buy MC for a C5 Corvette, perfect fit.

nate99

I understood that the fittings on the brake lines were a different size/thread. I could be wrong.

Frankenvette

Glad you asked this question. I've been thinking about this mod for years but never got around to investigating.

Jim

Shark Racer

I asked that question in Bee Jay's hood lightening thread - sounds like the M2903 and M2904 will both hook up to the vacuum booster in his 79, which is most likely the same as my 78 and your 81.

Marlin

So will the c5 bolt right up, thought it was metric? Looks like the C5 has a 1" bore is that ok?

The one the OP listed seems to have the same threads, but will it fit a standard powerbooster for a C3?

Will be doing a full brake rebuild come summer and was looking into this as well. Good topic.


EDIT

OK same thing I ran into before. The one the OP has listed has the same line fitting size, BUT they are backwards. The front and rear lines are flopped VS the C3.

Bee Jay

C5 and C6 Master Cylinders have metric fittings. Many have made them work with adapters. I was attempting to use these MCs with no adapters for my original C3 brake lines. I'm off to NAPA to pick up the 2904 now. Stay tuned.
Bee Jay

scottyp99

groovyjay said, and I quote, "perfect fit". If you need to make custom brake lines, it's not a "perfect fit".
I, for one, would love to get verification that the 2904 is virtually a bolt-it-in-and-go proposition. I would love to get rid of the big, heavy cast iron MC, without having to make custom brake lines. What is up with what Marlin says about the front and rear brake lines being flipped? What does this mean? Are the fittings flipflopped, in which case it wouldn't work, you'd need custom brake lines anyway. Does he mean the front of the MC controls the rear brakes? That would make the fittings wrong, and again, custom brake lines are in order.

Does anybody know if the 2903 MC fits the stock C3 brake booster? Do the original brake lines work? Is there a modern MC that will bolt to my factory brake booster, using stock brake lines, with no catches, or not? Once we get an affirmative answer to this question, it will be a very, very popular upgrade for those of us not running hydraboost (whatever that is), I think.


Scott

nate99

I think BeeJay is about to confirm. And then NAPA will wonder why all of these 2000 C2500 trucks had their master cylinders go out at once.

scottyp99

It sounds like BeeJay is running the Hydraboost (whatever that is, sounds like maybe a brake booster that uses something besides engine vacuum) are there differences between the Hydraboost and a stock C3 brake booster that would cause issues for people like me who want to keep the stock booster, just upgrade to a more modern, lighter MC? If BeeJay can verify what the deal is with the whole brake line fitting issue, that will help a lot, I guess. I will be keeping an eye on this thread, because if it turns out that this will work for me, and others like me, I want to get to NAPA before the rush!!!


Scott

Shark Racer

Hydroboost is a brand name for brake boosters that are powered by the flow of power steering fluid as opposed to a vacuum diaphragm, FYI.

They are pretty common on later truck models. My 06 Avalanche uses one.

(out of curiosity - to those who run one, does the brake pedal "kick" when you start your motor with your foot on the brake?)

wcsinx

I'm pretty sure Marlin is right about that. Just look at the size of the ports. The stock C3 MC has the larger fitting in the rear where that newer one has the larger port up front. Still though, this wouldn't be a difficult hurdle. There might be enough slack as is ... worst case you'd have to extend just one line a little.

But yeah, I would love to get rid of that lump of iron and that f'kin bailing wire cap.

scottyp99

So, the front brakes connect to the rear of the new MC, and the rear brakes connect to the front of the MC? Is this going to cause problems, or will it work just fine that way? Sorry if that sounds like a dumb question, but I have never hooked up the brakes backwards, have no idea what the consequences would be, if any.


Scott

scottyp99

Interesting!! What is the benefit? Big cam, not enough vacuum for brakes, run Hydroboost?


Scott

Frankenvette

Not a dumb question at all. Better to find out now. I'd like to hear the answer as well.

mrvette

The advantage is that is works, it actually stops the car, even with DOT5 fluid, like I have, and O ring SS calipers

so yes, the conversion is worth it, BUT if you have a overbore master cyl on now, get set to go through the windshield as your shark tips over it's nose and you land on your head, end over end....

yes, Virginia, the pedal change is that great....

I use a Pirate Jack racing m/cyl on my car for years now with the HB setup, stock bore dia, and it stops every time, nice pedal feel too, none of the 4' of travel and wonder if I can stop before the firewall feeling anymore....

I have posted in so many threads about the internal structure of the stock vac booster as having a design flaw, a spring between the input and output shafts, there is NO such spring in a HB unit....