Chevy Impala SS Forum banner
1 - 20 of 64 Posts

·
Premium Member
Joined
·
1,072 Posts
Discussion Starter · #1 ·
Ever since I bought the car years ago, the brake pedal pressure has been hard. I've replaced damn near everything including the booster and master cylinder. I know the car has both the stealth brake mods done, so I am wondering if these are the cause of the high pedal pressure. Any ideas? Its kind of the last thing to try....

TIA
 

·
Registered
Joined
·
1,865 Posts
I think we once talked about master cylinders before. There are 2 flavors. A large bore cylinder will give a hard pedal and a small bore. First time I drove a friend's 94 9C1, i thought her car lost the power assist. I run one now in the 91 with both brake mods and it's a stiff pedal. The mods wouldn't make it stiffer by themselves. If I didn't have a grippy set of FF pads, I suspect it be pretty hard stopping the car. So my suggestion would be try the other master cylinder or try a set of pads with an FF rating or higher. It would still be a firm pedal but you wouldn't have to push as hard to get it to stop. I'm using the Bendix Fleet Metlok. Great pad if you don't mind all the brake dust.
 

·
Registered
Joined
·
8,728 Posts
Other variables to throw in the mix could be:
the Kore brakes
froze rear pins (if used)
other PM

Without knowing more, I'd just go through and check that all 'manual' operation and cycling works/looks right, then eyeball for bent-jammed SYHK underneath from roadkill or shrapnel. Then a one-time grande 4-side bleed job.

I had the rear bolt mod only on my FWB, but it worked like a charm.
 
  • Like
Reactions: atlantadan

·
Premium Member
Joined
·
1,072 Posts
Discussion Starter · #4 ·
Among the things I replaced were both front calipers.... I may have to try (another) master cylinder and swap out the pads. I am currently running ceramics on all 4 corners.
 

·
Registered
Joined
·
1,725 Posts
For those of us that are not familiar with what brake parts you have front and back could you please post this info. Kits might change and some people mix and match.

For the beginners: To test brake system boost While parked with the engine off press the pedal to the floor several times the second or third press should feel different, it should feel hard. Keep your foot on the brake pedal and start the car. The pedal should go down with out any extra force and feel soft.

Have a look at this post:


What I get from it is that there may be differences between pedal geometry of: 9C1, Impala, Caprice, Wagon, ect. This and the different MC bore will cause different feel. Change the volume of the calipers and things change again. Miss match caliper volume ratios front to back will also change things.

If I understand correctly a smaller bore MC will give you more pedal travel. If short pedal travel means firm to you a smaller bore will help. IF firm means you are unhappy with excessive pedal force to stop you may be able to lower the pedal force by finding pads that have more friction. Usually soft, fast wearing, and shed dust.
 

·
Premium Member
Joined
·
1,072 Posts
Discussion Starter · #6 ·
The catch is that I can't stop the car quickly. Like in a panic-stop situation, it would end poorly. I am not sure what to do here other than try a different proportioning block. Barring that, the entire kit may have to come off and be replaced.
 

·
Premium Member
Joined
·
1,072 Posts
Discussion Starter · #7 ·
I purchased the Wildwood proportioning valve since the OEM one is NLA. Hopefully, this will bring my brakes back. If not, the entire Kore3 kit is going in the bin and will be replaced with the Wildwood set
 

·
Registered
Joined
·
11,737 Posts
I swapped in a hydroboost brake booster. The one from a 07 Tahoe with the flat plate from a 94 or earlier Astrovan is an easy install (you can get of the plates on ebay for $25). The only mod is to cut the eye off, and weld the one from the original vacuum booster on at 6 3/8 on center. I have that on my 91, and it works great. Hoses are bolt in from a mid 80s diesel, and T the return line. You can use the original MC.
 
  • Like
Reactions: atlantadan

·
Registered
Joined
·
1,865 Posts
I purchased the Wildwood proportioning valve since the OEM one is NLA. Hopefully, this will bring my brakes back. If not, the entire Kore3 kit is going in the bin and will be replaced with the Wildwood set
If it's going in the bin, i'll pay shipping for it :) Seriously though, is this the 325mm set up with corvette calipers? If so and you haven't changed the master cylinder to match them, you're significantly applying less pressure to the pads. This that would explain your problem after reviewing the various bore diameters.
 

·
Premium Member
Joined
·
755 Posts
If it's going in the bin, i'll pay shipping for it :) Seriously though, is this the 325mm set up with corvette calipers? If so and you haven't changed the master cylinder to match them, you're significantly applying less pressure to the pads. This that would explain your problem after reviewing the various bore diameters.
Can you walk me through that conclusion (less pressure (force?) to the pads)? I'm thinking that larger calipers and more piston area with no changes in master cylinder would result in more force clamping on the pads than the stock setup.
 

·
Premium Member
Joined
·
1,072 Posts
Discussion Starter · #11 ·
If it's going in the bin, i'll pay shipping for it :) Seriously though, is this the 325mm set up with corvette calipers? If so and you haven't changed the master cylinder to match them, you're significantly applying less pressure to the pads. This that would explain your problem after reviewing the various bore diameters.
I don't think I am being clear - I could literally apply every bit of strength to I've got to the pedal and there's zero chance the ABS would ever engage. Were the MC diameter the root cause, I would not be the only one posting with this issue after a brake upgrade. Navy Lifer would have made mention of this during all of his work, too.

And yes, the MC has been replaced with the cop version. The front calipers were also replaced.
 

·
Registered
Joined
·
8,728 Posts
I've read attention being paid to all the components except no word noticed yet about the vacuum circuit. Is there a way to disconnect / remove maybe the MC to make sure free cycle of pedal mechanical assy to rule out mech. bind?
 

·
Registered
Joined
·
1,865 Posts
I don't think I am being clear - I could literally apply every bit of strength to I've got to the pedal and there's zero chance the ABS would ever engage. Were the MC diameter the root cause, I would not be the only one posting with this issue after a brake upgrade. Navy Lifer would have made mention of this during all of his work, too.

And yes, the MC has been replaced with the cop version. The front calipers were also replaced.
Not everybody's an engineer ;)
Caprice calipers are 2.9" diameter = 6.6sqin
corvette calipers are (2) 41mm = 4.1sqin
corvette runs a 1" MC for an effective 5.2:1 'leverage' increase
caprice with the 1.125" has a 6.64:1 ratio (664 lbs of force on pads for 100 lbs of force on master cylinder)
caprice with 1.25 9C1 caliper is down to 5.38:1 ratio. Still higher than a stock corvette.
There probably pedal leverage difference between vette and caprice, but you haven't changed the pedal. So, you have this:

Put a corvette caliper with a 9C1 MC and you have only a 3.33:1 ratio. You are now generating HALF the force at the pads! There's a significant feel difference going between 9C1 and non. Can't fathom your situation.
 

·
Registered
Joined
·
1,865 Posts
Can you walk me through that conclusion (less pressure (force?) to the pads)? I'm thinking that larger calipers and more piston area with no changes in master cylinder would result in more force clamping on the pads than the stock setup.
See my response to Dan. Your assumption is right. It's just that the Z05/Z06 calipers have smaller piston area than the caprice's. And why it uses a smaller bore MC to compensate.
 

·
Premium Member
Joined
·
1,072 Posts
Discussion Starter · #15 · (Edited)
I appreciate your brain-power here, for sure. However, I find it very hard to believe that out of the hundreds? Thousands? of people that have installed kits like mine, nobody has mentioned the conversion making the car borderline unsafe to drive. Navy Lifer says that he's running the exact same system as mine and his brakes are acceptable. Logically, if this is 'par for the course' when doing a bbk, one would expect to find many threads with this same concern. That is not the case.

The pedal feels like it just stops and that's all the movement you're going to get. It travels a certain distance and then it feels like you're pushing against a wall.

Your data above is interesting and it brings up the question of why anyone would go to the 'vette bbk if the Caprice calipers are larger. That's big-brain stuff, and I lack the computational power.

I am fully prepared to eat my share of crow if the Widlwood proportioning valve doesn't fix the issue. I'm currently eyeballing the Corvette MC at Advance... $70 and I have a credit so it may be worth a shot to install at the same time as the proportioning valve.

96 Black, all the vac lines are new and the draw is 100% to spec. I also made sure nothing is actually stopping the pedal travel (good suggestion, btw!) and everything is good.
 

·
Premium Member
Joined
·
1,687 Posts
Dan, Did this happen before you took your car to the body shop or is it since? I read all your past posts since I joined here, and never saw where you mentioned this prior. (That I recall) You have had troubles with that shop. Not being a conspiracy theorist, it's the cop in me. I have to wonder about the possibility.

Mark: Snowman-33
 

·
Premium Member
Joined
·
1,072 Posts
Discussion Starter · #17 ·
The stiff pedal has been an issue since I got the car. It's just now become more of an annoyance than I am willing to deal with. I believe I have posted about it before, but may not have.

I just ordered the C5 master cylinder and will install it at the same time as the proportioning valve. I suspect that between the two, I'll get the brakes I need
 

·
Registered
Joined
·
1,865 Posts
I appreciate your brain-power here, for sure. However, I find it very hard to believe that out of the hundreds? Thousands? of people that have installed kits like mine, nobody has mentioned the conversion making the car borderline unsafe to drive. Navy Lifer says that he's running the exact same system as mine and his brakes are acceptable. Logically, if this is 'par for the course' when doing a bbk, one would expect to find many threads with this same concern. That is not the case.

The pedal feels like it just stops and that's all the movement you're going to get. It travels a certain distance and then it feels like you're pushing against a wall.

Your data above is interesting and it brings up the question of why anyone would go to the 'vette bbk if the Caprice calipers are larger. That's big-brain stuff, and I lack the computational power.

I am fully prepared to eat my share of crow if the Widlwood proportioning valve doesn't fix the issue. I'm currently eyeballing the Corvette MC at Advance... $70 and I have a credit so it may be worth a shot to install at the same time as the proportioning valve.

96 Black, all the vac lines are new and the draw is 100% to spec. I also made sure nothing is actually stopping the pedal travel (good suggestion, btw!) and everything is good.
No worries. I just go where the math takes me. How far does the pedal travel before the hard stop? You might be bottoming out the MC if there's an air pocket somewhere.
IF you should try the vette MC at some point, looking at an 05, looks like you'd have to reflare one line as the vette has two identical sized ports assuming the bolt hole spacing is the same as a caprice's.
 

·
Premium Member
Joined
·
1,072 Posts
Discussion Starter · #20 ·
No worries. I just go where the math takes me. How far does the pedal travel before the hard stop? You might be bottoming out the MC if there's an air pocket somewhere.
IF you should try the vette MC at some point, looking at an 05, looks like you'd have to reflare one line as the vette has two identical sized ports assuming the bolt hole spacing is the same as a caprice's.
I bought a 99, since it also has the 1" bore. I've got a flare kit and a tubing bender . I won't know for sure until I get in there, though.

For the group's knowledge, it'd be nice to do one at a time to see which affects the most. Just suggestion.
Yeah, but that is two complete bleeding sessions. Even with the Motive bleeder, that's a pain. I dunno.... If I mod a spare cap it'll be easy. Let me see what I see when I dig in
 
1 - 20 of 64 Posts
Top