Stock Brake Problems, 20mm master cylinder? Options

[Rog914]

I'm putting new dics, rebuilt calipers & new brake hoses (all stock, no big brakes)on my 914. So I thought I'd check and see what size master cylinder I have. Clean off the bottom of the master cylinder and see a 20 cast on the body of the MC.
The brakes were always bad, meaning hard to stop. You really had stand on the brakes hard to stop.
Would a 17mm MC make my brakes feel like power brakes compaired to the 20mm MC on it now? Would it make that much of a difference?

Ralph

74 2.0

[grasshopper]

a 17 is going to make your brakes feel less powerful. unless theres a prob with a 20mm keep it.

[SLITS]

The 20 should be between the mounting flange and the push rod end of the MC. If you didn't take it out of the car, it's a casting number on the main body.

If it were a 20, it would be off a MB. and the suspension carrier tube would have been dented to clear the end of the MC as I remember.

[bd1308]

a 20mm is going to have more mechanical advantage over the fluid in the system.....which translates into more power for equal travel. now you mentioned the brakes don't work well--what are you comparing it to?

my jeep stopped on a dime.....but vac assist brakes makes a world of difference.

[Rog914]

Britt, I had a 73 1.7 25 years ago with stock brakes, I could lock them up at low speeds. This 74 I have now I can't lock up the brakes at all. I have to be very careful when I drive and not get too close behind anybody. I can go 5 or 10 mph and put everything I got into with it only slowing down, that's with me pushing so hard on the brakes it feels like I'm deforming the brake petal assembly. The 73 1.7 would have locked the brakes up. That's what I'm comparing it to.

Ralph

74 2.0

[bd1308]

sounds like what i was doing yesterday at Al's house. We fixed my right-front caliper and we bled (he did--i watched) the brakes and i took off and i was doing what you were doing......pushing with all of my might and the car's brakes just plain sucked.

Sounds like you have some air in your brake system.....bleed bleed bleed. We did my car twice.....second time found fairly large amounts of air in the calipers.....and air can compress when forced to, leading to your braking problems. Remove the air, you remove the spongyness of the brakes and the lame effectiveness.

[Rog914]

Britt, I bought one of those "Power Bleeder" bottle. POS the gauge broke right off to start with. So I bought a set of 4 "Speed Bleeders". Best investment for Brake bleeding.

Ralph

74 2.0

[Joe Ricard]

NO NO NO. the bigger the diameter of the M/C bore the LESS mechanical/Hydraulic advantage you have.

20mm M/C will require alot of force to achieve the same amount of pressure @ the caliper piston. compared to the stock 17mm M/C.

Last night I actually did slide a tire on the course. Comming in way to hot and started braking WAY to late. Hey I was tired as it was near Mid night before we finished my heat. (anyway)
Bone stock 914-4 brakes with better pads. I think Porterfield R4S. Yea pedal travels a ways but it stops without drama every time.

[bd1308]

that was therory #2.....

anyway, my brakes are super when compared to before--when i only had three fully-functioning calipers.

[mihai914]

Why don't you wait to see what the brakes will feel like after all of your changes before commiting to buying a new master. Worst case scenario you'll end up changing the master anyway and will have to bleed the brakes again.

Also always keep in mind that tires do the braking, good tires will not lock up, shitty bald tires will always do it, but you're obviously referring to poor performance.

Oh yes last thing, make sure you adjust the rear calipers to .004 inches and not .008 as stated in the books, poor adjustment will give poor braking performance.

[bd1308]

QUOTE (mihai914 @ Jul 17 2005, 08:46 PM)

Oh yes last thing, make sure you adjust the rear calipers to .004 inches and not .008 as stated in the books, poor adjustment will give poor braking performance.

why? Just a hunch or do you have something to use to back up the statement with?

[mihai914]

Cap'n Crusty keeps on hammering this one in to people's mind along with the stainless brake lines no no. And there's probably a TSB somewhere.

[SLITS]

QUOTE (bd1308 @ Jul 17 2005, 07:49 PM)

QUOTE (mihai914 @ Jul 17 2005, 08:46 PM) Oh yes last thing, make sure you adjust the rear calipers to .004 inches and not .008 as stated in the books, poor adjustment will give poor braking performance. why? Just a hunch or do you have something to use to back up the statement with?

Yep, from what I hear a lot of us do it....much better brake feel and performance

[sgomes]

I wouldn't have believed it if I didn't do it. It's hard to understand how .004" can make a difference but ...

[Dave_Darling]

I figured out the 0.004" the first time I tried using the clearance in Haynes. "Hey, this 0.008" adjustment SUCKS!"

BTW, if you "lose your way" with the MC sizing, you can take it all back to the basic units everything is measured in.

Your foot exerts a force on the pedal, measured in pounds. After you deal with the leverage from the pedal itself, (a multiple of) this force presses on the master cylinder piston. That creates a pressure in the brake system, in PSI--Pounds per Square Inch. That is, lbs/sq in. So you divide by the cross-sectional area of the MC. A larger MC results in a lower pressure for a given force on the pedal.

When you get to the caliper, a similar thing happens, but the other way around. The caliper pistons take that pressure (PSI) and convert it into a force pushing the brake pad against the rotor (lbs). So you multiply by the cross-sectional area of the caliper piston.

With a larger MC, the pedal moves less and feels harder. You have to push harder to get the same effect. Which is almost exactly like using a shorter lever. Only, in this case, the "leverage" we're looking at is "hydraulic advantage", not the "mechanical advantage" of an actual lever.

--DD

[Cap'n Krusty]

QUOTE (bd1308 @ Jul 17 2005, 06:49 PM)

QUOTE (mihai914 @ Jul 17 2005, 08:46 PM) Oh yes last thing, make sure you adjust the rear calipers to .004 inches and not .008 as stated in the books, poor adjustment will give poor braking performance. why? Just a hunch or do you have something to use to back up the statement with?

It's true. They told us at P car school the book is wrong, but the book never got changed. I can back that up with 32 years 914 experience, should you need emperical evidence. The Cap'n

[Eric_Shea]

The MC size is on "top" of the unit close to where it mounts. My guess is it's actually a 17mm (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

[maf914]

QUOTE (Dave_Darling @ Jul 17 2005, 09:48 PM)

Your foot exerts a force on the pedal, measured in pounds. After you deal with the leverage from the pedal itself, (a multiple of) this force presses on the master cylinder piston. That creates a pressure in the brake system, in PSI--Pounds per Square Inch. That is, lbs/sq in. So you divide by the cross-sectional area of the MC. A larger MC results in a lower pressure for a given force on the pedal.

Dave is correct. A 17mm MC has a cross sectional area of 0.352 sq. in. A 19mm MC has an area of 0.440 sq. in. If you apply a 100 pound force to the cylinder rods you will generate 284 psi with the 17mm and 227 psi with the 19mm. The larger the cylinder bore the more force it will take to generate the same brake line pressure.

The smaller MC will generate less volume during the stroke, so it will take a longer stroke to deliver the same amount of fluid as the larger MC.

Better hit the leg press machine at the gym! (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

[zymurgist]

I was told when I had the master cylinder rebuilt for the Corvette, that I should bench bleed the master cylinder before installing it. It doesn't help to get all the air out of the lines if there are a few bubbles trapped in the master cylinder. Could this possibly help your situation?