Improving pedal feel

[ULTM8Z]

Ok, so last night I fabbed up a new booster pin from some steel stock I got from the hardware store. Didn't feel comfortable driving around with that little spacer.

The initial pedal play seems to be gone, but there's really no improvement in the late pedal feel. With the car off, my foot feels a very hard brake pedal with very little movement, but as soon as the car starts it turns into a sponge and my foot sinks about 1/2 way to the floor.

Like I was saying the original post, the amount of assist I'm getting doesn't seem to translate into the amount of braking force you'd expect.

So I'm starting to think about the 7/8" bore again... everything I'm reading indicates that the smaller bore will generate more line pressure.

Also as I posted earlier, with my 3.5:1 pedal ratio and C4 Corvette calipers, I should be using the 7/8" bore in order to match the 88-91 Corvette set up.

The only thing is I guess the pedal will feel even softer than it is now.


One other thing to note is that I was evidently incorrect on saying it's a 9" dual diaphragm. I had tried to measure it in the car with the master cylinder on it and with the G-braces around it. When I pulled it off the car yesterday and measured it, the outer case is actually measuring ~8" in diameter. I'm assuming that indicates I have 8" diaphragms.

Does anyone know what the C4 Corvette booster size is? I'd like to compare the amount of assist I'm getting with the Vette.

At my 12 inHg vacuum (~6 psi) at idle, if I'm doing my math right with the area available on a 8" dual diaphragm, I should be getting about 600 lbs of assist off the booster.

EDIT:

So doing a little looking up online...

Vette booster diameter is either 8.875 or 9.75 according to NAPA...

Manifold vacuum is about 17 inHg in gear for a L98 near as I can tell

That means they're getting between 500 and 600 lb of assist. AGain, if my math is correct..

So that means with a 7/8" bore (as compared to my 15/16") they're getting an even softer pedal than I am? I'm confused...

[Skip Fix]

Why not look at the 79-81 booster dual diaphragm and made for an F body .

[ULTM8Z]

The diameter is critical since my current booster just barely fits between the G-brace cowl braces.

I think what I'll do is try the 7/8" booster.

Maybe if the pedal feels too soft, I can get a little stiffer return spring for the pedal arm and try to tune it that way.

[Skip Fix]

The TA booster is 9" and I bet would have no issues with the G braces, I bet they were designed to clear the 11" disc drum booster lots of guys use or he would have limited market. My old Herb Adams G braces sure did.

Pedal feeling soft is a function of pedal ratio, booster assist, MC diameter and any air in system. Return spring not really the way to tune it.

[ULTM8Z]

Understood.

I ordered the 7/8" MC last night. The 1980 GM cars like the Monza, Malibu, some of the Buicks, etc with manual brakes ran a 7/8 bore with plastic reservoir but still had the SAE sizes on the fittings, exactly what I was looking for. It'll save a little weight in the process.

Any rate, once my hydraulic system and pedal ratio match the 88-91 Vette specs, then I know that part will be a known good. The booster will be the next thing to look at if it's not quite where I want it.

The Russel speed bleeders will take care of any air in the system... damn those things make it easy... I don't even have to take the wheels off the car... just reach around the tire, loosen the bleeder screw, hook up the hose and pump the pedal 4-5 times. What I do is hook up my camera on a tri-pod pointing at the hose. After 4-5 pumps (and ensuring the reservoir is still full) I review the footage and see if there were any more air bubbles coming out of the last couple of pumps. Works out fantastic.

After that, if I think it needs any fine tuning for feel, I may look at the return spring.

[Skip Fix]

Cool way to bleed with the camera! I never seem to have help when I need it. Good luck. I think there was a manual S-10 MC that had standard fittings and on the correct side.

I'm having an issue with my 81 TA after freshening brakes myself. Used some old organic Bendix FF pads on the rear when I did it first and they would lock up before the old front. Redid the front new calipers, new Hawk HPS pads and the rear still locks up. So I must have a prop valve issue with the factory prop(but it didn't show with the bad rears on it). I wouldn't think the pads are making that much difference. Have a pressure gauge I'll put on the calipers to see if it is a pressure issue to jut put a T and a rear adjustable valve on it vs trying all new pads.

[ULTM8Z]

Ok, I'm totally stumped...

I put the 7/8" MC on and it's now worse than before in terms of stopping power. WTF? I'm literally nearly to the floor before there's any significant grab.

I've already had the thing on and off three times re-bleeding the MC and re-bleeding the brakes.
I expected more travel, but I thought the smaller 7/8" was supposed provide more clamping force than the 15/16" overall.

No idea what I'm doing wrong here...

I'm definitely not lacking any boost assist... I'd think if that was the case, I'd feel a lot firmer pedal as soon as I touched it.

Could the prop valve be bad? What would be the symptoms of something like that?

[Skip Fix]

Smaller does give more pressure pre input on pedal, but it might not move the volume needed by the calipers without a lot of travel-why more travel.

Larger MC will move more volume but take more pressure from the pedal to get the same line pressure. Most of the power brake disc/drum cars (and the 79-81 TS 4 WD) used a 1 1/8 MC. But they did use a front caliper with a 2 15/16" piston(2.5 rear pistons on the TAs).

[ULTM8Z]

Well, I ordered a 1" master cylinder just now. Guess we'll see how that one works out.

[ULTM8Z]

So, I'm not sure what actually did the trick here, but the brakes feel GREAT now...

I ended up going back to my original Baer MC ironically, and plumbed the same way after playing a guessing game for weeks... Rear port to front brakes and front port to rear brakes.

https://www.pro-touring.com/threads/...ediately/page2

Then, between the new proportioning valve, new 11" booster, and the bleeding technique I finally settled on...

http://nastyz28.com/threads/one-man-...hnique.317185/

The brakes finally feel like they should have felt all along... pedal response is razor sharp and just as firm as I like.

In reality, I knew I was on the right track after having installed the 1" MC. Late pedal was gone and the grab was good. Just the pedal effort was a bit too much. Which made me go back to my 15/16"... It's perfect now.

I may try some Hawk or Stop Tech pads for optimization later on, but right now it's great.

So much thanks to everyone who chimed in to help out!! (especially Skip Fix who I used as a sounding board quite a bit over the course of several threads).

Hallelujia this saga is finally behind me...

[CarlC]

I'm betting that air is still in the system, likely in the MC.

A small piston MC will make for an easier pedal with a longer stroke length. A larger piston will make for a harder pedal but less stroke length. If you're getting a different result then either the components are bad or there is air in the system.

Did you thoroughly bench bleed the MC multiple times? I've chased that rabbit many times only to find out it's air in the MC. I now bench bleed at least two times with a lunch break between. Speed bleeders are no replacement for bench bleeding.

Even new MC's can be bad. I've gone to using only new GM MC's due to poor aftermarket quality that cost me a full day worth of chasing what ended up being a bad part.

Mark Stielow has a video of a tool he made that vacuum/pressure fills the brake system. After seeing how it works it's a no-brainer as long as an OEM GM MC is used.

https://www.youtube.com/watch?v=z0ka1vbtePg
https://www.youtube.com/watch?v=T-l6HCBOInE

[ULTM8Z]

I'm kinda leaning toward air in the system as well.

Yeah, I bench bled the master cylinder. And as you say, I let it sit for a little bit and then came back and bled it again to make sure no residual air was in there. This was for both the 1" and 15/16", which gave me a good apples to apples comparison... and yes, the 1" was a stiffer pedal than the 15/16".

[Skip Fix]

The Strange MC I have on the Camaro took forever to get the air out bench bleeding. Had to do the block off the ports to see there was still air somewhere. Then the Line Lok took forever to bleed also.

[Pete68]

Its tough to mix and match these components. Its good to get an entire system. Thats why I recommend using all C5 or C6 Corvette parts in the whole swap. On my car the only thing original are the steel lines. ALL the other parts are from the C5 Corvette, including master cylinder and booster. Pedal feel, initial grab, pedal pressure, feedback, modulation, are all great. My daily driver is a 911 Carrera S and its comparable feel, does not brake as well of course (because 50 year old chassis), but pedal feel is similar. Good luck.