Help with Brakes, Wrong MC?

[JLMounce]

Over the last week I went through my entire brake system on my 1969 Firebird. The system has functioned well for many years now. It was never what I would call fantastic, but you could drive it hard and the brakes let you do what you wanted with the car. It was a basic setup consisting of factory style 11" rotors at all 4 corners, OE D52 calipers in the front and OE D154 calipers in the rear. Everything had gotten a good bit of rust on it, so I opted for the Wilwood D52 and D154 replacement calipers.

After wiring wheeling the discs and coating the hats for looks, I installed the new calipers and bled the system using a vacuum bleeder.

I completed the bedding procedure and I'm left pretty disappointed with the brake performance afterwards. I have a firm pedal with positive feedback and under normal braking the car stops as expect. Under hard or near panic braking, there's just not much more there. As the car starts to slow rapidly, I reach the limit of the brake pedal's travel and I'm unable to get to the point where locking the wheels is possible.

Here's a rundown of the entire system.

9" single diaphram booster
Engine makes 13" of vacuum at idle
1" bore MC
2" dual piston D52's in the front
1.12" dual piston D154's in the rear
Clevis attached to the bottom hole in the pedal. I believe Camaro/Firebird factory lower hole is 3.8:1 ratio

The previous calipers had the 2.75" pistons in the front and 2" pistons in the rear. 5.94 in^2 for the front and 3.14 in^2 in the rear.

New calipers are 6.28 in^2 in the front and 1.98 in^2 in the rear.

The way the pedal acts, it almost has me thinking the MC is undersized for caliper area, but I wouldn't think this small amount of increase in the front, and reduction in the rear would be that noticeable. The brakes are not hard to operate, so a little more pedal effort wouldn't be a deal breaker is a 1 1/8" MC would give me the travel necessary for proper function again.

I've thought about moving the clevis up to the top hole in the pedal as well, but am fearful that with the booster in place, it'll be too twitchy.

What's everyone's thoughts on this? I'm going to bleed the system another time tonight after it's been driven at sat for a bit to see if I just don't have a small amount of air, but I'm pretty positive it's bled properly.

Thanks in advance!

[stab6902]

I'm assuming your booster has a threaded input rod. Did you setup the clevis so the pedal is at the stock "height" (with your foot off the pedal) so you have maximum pedal travel available?

The other easy fix could be air in the lines as you mentioned.

Moving the clevis to the top hole would be a step in the wrong direction - you'd get even less input rod travel.

I think there's a good chance you'll have to go to the 1-1/8" master cylinder. That extra 1/8" might not seem like much, but it makes over a 26% difference area wise. It's strange that your brake performance was good before though. Are your new brake pads the same compound?

[JLMounce]

I'm assuming your booster has a threaded input rod. Did you setup the clevis so the pedal is at the stock "height" (with your foot off the pedal) so you have maximum pedal travel available?

The other easy fix could be air in the lines as you mentioned.

Moving the clevis to the top hole would be a step in the wrong direction - you'd get even less input rod travel.

I think there's a good chance you'll have to go to the 1-1/8" master cylinder. That extra 1/8" might not seem like much, but it makes over a 26% difference area wise. It's strange that your brake performance was good before though. Are your new brake pads the same compound?

That was kind of my thinking too in regards to the pushrod location.

I didn't think about checking the clevis to see if it's threaded. The original kit was put on the car back in 1999 from an original series MP Brake kit. The rears were added in 2008 when I did the moser rear end. I can't say that I recall that far back on the install of the front system which had the booster with it. I'll give that a check tonight as well.

The pads now are Wilwood's BP10 that come with these calipers. The previous pads were the basic pad that came with the old OE style calipers. I don't believe they were ever a performance pad, just a basic OE replacement. These new pads should theoretically have a bit more bite, I would think.

[morrisclassic]

JLMounce,

I would call Tobin at Kore 3.
He is the brake man. He will be able to give you a
Real fix the first time.

Billy

Morris Classic Concepts.

[JLMounce]

JLMounce,

I would call Tobin at Kore 3.
He is the brake man. He will be able to give you a
Real fix the first time.

Billy

Morris Classic Concepts.

Thanks. I'm going to do a bit more testing on my end first before I take up somebody's time at their job.

I did inspect the clevis last night and it's installed correctly, with the pedal against its positive stop in the hanger. I do note however that I have about a 1/4" of pedal travel before you can feel the pushrod contact the MC bore. It may be that the pushrod/bore clearance is out of spec. So next stop is going to be removing the master to check that before I spend any money on new parts.

I'll bench bleed again, then do the entire system once more and see where I land.

[Hotwire]

Bump the master up to 1 1/8". I'm running 2000 Camaro disc brakes (cheap LS1 upgrade) on my 64 chevelle, close setup to what you have. I used a 1980 Corvette master, has the shallow pin to match the booster I'm using and 1 1/8" bore. Pedal feel is just like stock and it works very well.

Internet says I have Front 4.81^2in and Rear 2.46^2in for piston area.

One other thing that will probably come up, did you bench bleed the master? How old are your brake hoses (including frame to axle hose)?

[dontlifttoshift]

Definitely check booster to MC clearance first.

Is the pedal bottoming on the floor? The other replies make me think it is but I don't get that out of anything the OP wrote. If it is not, then a 1.125" master is only going to make the problem worse.

Do you still have the old rear calipers? Put them back on without changing anything else.

Semi related, which reservoir is hooked to the rear brakes?

[JLMounce]

Yes the pedal bottoms to the floor before there's enough clamping force to lock the brakes up.

I did keep the old calipers in case.

The MC is the original unit supplied by Master Power for the front disc conversion. It's a standard Corvette style MC with dual reservoir. Rear reservoir feeds the rear proportioning portion of a disc/disc combination valve.

[Hotwire]

I'm running 255/40/17 all the way around, no distribution block. Front reservoir goes to a line lock, dual lines out to front tires. Rear has wilwood adjustable proportioning valve on it.

Was headed to GA from NC to mess with my friends on the Suches Loop, I was in the fast lane and someone just cut right over to avoid traffic, I didn't have time to let off the pedal to regain traction. This was the result if you're looking for evidence of a working system.

https://static1.pt-content.com/images/pt/2022/04/51246249822_23acf382a7_c-1.jpg

Did you bench bleed the master? How old are your brake lines including the frame to rear hose?

[JLMounce]

The master was bench bled in 2008 when I installed the rear disc setup. The new combination valve I used required that to be done.

Rear lines were redone in 2008 and I replaced the soft line from the rear to the axle last year. Front lines were done circa 2000. All factory sized and bent.

[dhutton]

I would bleed them again using the conventional method. I’ve had some funny things happen with vacuum bleeders.

Don

[JLMounce]

Mostly good news this weekend. I've spent a couple hours working on traditionally bleeding the system with my wife's help. Tried to get the calipers as vertical as possible while still having some of the rotor act as a spacer. There was definitely a small amount of air in a couple of the calipers.

The road test went much better. I am no able to lock up the wheels towards the end of the pedal travel. They will only lock up as the car slows, not at speed. This is leading me to believe that I have the air out of the system, but that the MC sizing is still probably wrong for the system.

There's one major issue that puts an apples to apples comparison with my old calipers in question. I recently upgraded to a set of Azenis 615K+ tires all around from the BFG Sport Comp 2 rubber it had previously, which was also 7 years old. I expect that the added levels of grip contribute here.

She'll definitely be good for the driving season now and I'll address the master cylinder towards the end of the year. I'd still love to hear input if you have it and thanks everyone for the help.

[TheBandit]

That's great news. In my 70 Nova with the factory big single piston discs / drum rear the master was 1-1/8" with 11" booster. You might also check to make sure you are using the right hole in the pedal arm and that the booster pin is adjusted properly. Just some ideas.

[JLMounce]

Bringing this thread back from the dead. While the experience last year had gotten better, it was never quite as good as with the standard GM OE calipers front and rear. After a few changes this year, the braking performance actually got worse and it's keeping me from enjoying this car.

After having a 4l80e put into the car, the brakes were terrible. I had to be very cautious getting the car home. At this point I set out trying to determine where the issues were. I found that the front seal in the 20 year old booster was cracked. Although tests showed the booster was working, I always felt it was too small for the engine vacuum I have available.

The first thing I did was replace the booster with a Tuff Stuff dual 9" unit and a new AC Delco c3 style master cylinder in a 1 1/8" bore size. At the same time, I was taking no chances on old brake lines, so the car got new lines front to rear. Stock sized 3/16" front stainless lines and went to a 3/16" rear line over the 1/4" factory size.

Bled everything a number of times and the braking performance got a bit better, but the same issue of having an immediate pedal with nothing after was still there. My next suspect was the disc/disc combination valve. I replaced that with at speedway adjustable proportioning valve block.

This gave me more adjustability of the rear, but ultimately didn't change the characteristic of a very immediate pedal, followed by lackluster stopping power as you stomp on the pedal. In fact, the dual 9" booster has made that situation worse. The brake pedal for the first inch of travel is extremely touching, well beyond what I would describe as an over-boosted situation. The next bit of travel adds no additional breaking force. It's all done in that first inch or so of travel and the pedal feel is beyond atrocious. I would describe it akin to the first time you try left foot braking, but your foot has had localized anesthesia.

I've sucked, pumped and pushed about 3 liters of brake fluid through the system using every bleeding method I know of. Traditional 2 person method, gravity bleed, vacuum bleed, reverse pressure bleed.

I run custom offset 17" TT2's on the car and there is a barrel hump where the spokes integrate to the hoop that prevents any of the corvette based brake systems or the larger rotor/caliper systems. So changing the brakes all together is a wheel/tire change as well.

I'm really just looking for any and all ideas that might be occurring here. I've spoken extensively with Tobin and he's been extremely helpful, but nothing has really worked and because I can't really buy his product, I don't want to waste his time.

Thanks much!

[dontlifttoshift]

Do you still have the old rear calipers? Put them back on without changing anything else.

Semi related, which reservoir is hooked to the rear brakes?

Your bias is way out of whack. As it sits, you need three times the volume for the front brakes as the rear. Since the rears are first inline on the master cylinder once they build pressure, that's it for pedal travel and you can't build any more pressure in the fronts. I would downsize the fronts or upsize the rears to get closer to 2:1 on the area, like Hotwires specs that he posted above. A little more in the rear is okay, that's what the prop valve is for. If you don't have your old ones anymore, Wilwood has the D154R in a 3.14 area that should work better than what you have.



...208598


Problem number two is you don't have enough vacuum to run that booster. Install this. https://leedbrakes.com/p-33752-leed-brakes-bandit-series-vacuum-pump-kits.html

[JLMounce]

Your bias is way out of whack. As it sits, you need three times the volume for the front brakes as the rear. Since the rears are first inline on the master cylinder once they build pressure, that's it for pedal travel and you can't build any more pressure in the fronts. I would downsize the fronts or upsize the rears to get closer to 2:1 on the area, like Hotwires specs that he posted above. A little more in the rear is okay, that's what the prop valve is for. If you don't have your old ones anymore, Wilwood has the D154R in a 3.14 area that should work better than what you have.



...208598


Problem number two is you don't have enough vacuum to run that booster. Install this. https://leedbrakes.com/p-33752-leed-brakes-bandit-series-vacuum-pump-kits.html

Thanks, this does make a little sense and is what I’ve been trying somewhat unsuccessfully to research. I did speak with Leed about that product, but they didn’t advise its use since I am at close to 5000 ft of altitude. They said it would never reach its pressure cutoff and burn up the pump.

that being said, atmospheric pressure here is much less than 14.7:1. Am I correct in assuming that for power break operation it’s important to look at the ratio of vacuum to atmosphere rather than specifically total vacuum?

I actually questioned Wilwood on their rear caliper size recommendation because the GM style stuff that I’ve had worked okay.

im wondering if I should change these out completely to their d52r in the front and d154r in the rear. Or just go back to the ugly GM cast iron stuff altogether.

Alternatively, I recall seeing this before; what would happen if I plumbed the primary circuit to the front of my distribution block and the secondary circuit to the rear with the proportioning valve?

[JLMounce]

It looks like the standard D154 calipers have a couple options for upsizing.

This 2 piston caliper has an upsized piston area of 4.12sq/in
https://wilwood.com/Calipers/CaliperProd?itemno=120-11872-RD&appid=0

Or, this single piston caliper has a total of 4.90sp/in
https://wilwood.com/Calipers/CaliperProd?itemno=120-11870-RD&appid=0

They are the same price and take the same amount of work to replace. Knowing that, with my setup, would you opt for the dual or single piston? The front's have a piston area of 6.28 sq/in.

[JLMounce]

On dontlifttoshift's advice above, I went ahead and purchased a set of new rear calipers. When with the 4.12sq/in dual piston D154's from wilwood since they look the same.

big improvement in braking performance. I need to spend some time adjusting the proportioning valve this weekend as the rears lock up much too quickly.

I still can't lock up the front's, but I suspect that the amount of pressure they were seeing was so low as to not have allowed bedding of the pads. The car will definitely stop now however with at least the same performance it had prior to this caliper change.

I still don't like how the pedal feels, there's just not much feedback and it still feels overboosted. It functions more linearly now as you would expect however.

Thank you everyone for the help.

[JPnTX]

Have you considered an iBooster? You'd see plenty of line pressure with no vacuum requirements. 209326

[dontlifttoshift]

Aren't you the guy that sells the conversion kits? Just be honest about it and bring the tech instead of whatever it is you are doing now.

[JPnTX]

Sorry you took offense to my suggestion. I am of the opinion that an iBooster, regardless of where he purchases it, will not only resolve his issue, but improve overall braking performance. I have the same calipers he referenced on one of my own vehicles and it works very well. There's enough line pressure and adjustability with that setup to work with anything from disc/drum to 6 pots all around.

[dontlifttoshift]

LoL..... definitely not offended. The point you missed is that if you are going to slyly pimp your stuff bring the tech and help everyone out.

[dhutton]

lol..... Definitely not offended. The point you missed is that if you are going to slyly pimp your stuff bring the tech and help everyone out.
x2!

[JPnTX]

Ok, gonna have to change my username to Sly Pimp! Too funny. We can agree to disagree on the approach but my intention was exactly what you described, to promote discussion but do believe that a rising tide raises all boats.
For most, the concept of electric brakes is foreign and a retrofit is not even on the radar. It’s a tough segue. That said I have quite a bit of experience with these things and glad to answer any questions.

To start, there are two generations of iBooster, each with pluses and minuses from a retrofit perspective but both are relatively easy to install. Gen 1 booster I prefer is from 16-20 Tesla Model S and Gen 2 from 18-20 Accord turbo. Both work very well for retrofit and are readily available.