Manual brake pedal question

[sackracing]

I'm switching from vacuum brakes to manual brakes and was wondering if the vacuum brake pedal will work without modification. Anyone know?

 

[RmvBfrFlght]

I'm switching from vacuum brakes to manual brakes and was wondering if the vacuum brake pedal will work without modification. Anyone know?

Yes & no. The upper mounting hole for the pedal to M/C rod is present. But there isn't a stud in it. I used a 3/8" bolt (grade 8) and a thin nut as the stud. Used a 3/8" heim end for the rod end to the M/C. And a 3/8" grade 8 bolt for the rod. Cut the head off and tapered the end.

You have to be careful about the length of the stud/bolt. Too long and it will hit the bracket. So it is best to pull the bracket with the pedal and do all of this on the bench. The bracket is almost out with the booster unbolted. Just one additional bolt at the very top of the bracket and it is also out.

RemoveBeforeFlight

 

[sackracing]

I buying this kit:
PA Racing

It comes with the heims joint and rod, so I just need to move up to the upper mounting hole?

 

[RmvBfrFlght]

I buying this kit:
PA Racing

It comes with the heims joint and rod, so I just need to move up to the upper mounting hole?

Yes. Looks like the bolt is also included.

The thing I'm not sure about with that kit is the 1-1/8" bore M/C. That isn't going to create much line pressure unless you 2-foot the brake pedal.

With the stock brakes a 15/16" bore will be better. With LS1 disc's all around I went with a 7/8" bore M/C. The 1" M/C that was one there didn't cut it.

RemoveBeforeFlight

 

[Scott Atk]

If you saved the PM pedal, that has a decent ratio for manual brakes.

 

[Scott Atk]

That kit looks nice but you can do it for like $30. Parts store MC from 90's S-10 with manual brakes (15/16" bore). You can use the firewall adapter from your PM brakes. There's a couple threads on the subject. I'll see what I can find.

 

[S2V6RACER]

Yes. Looks like the bolt is also included.

The thing I'm not sure about with that kit is the 1-1/8" bore M/C. That isn't going to create much line pressure unless you 2-foot the brake pedal.

With the stock brakes a 15/16" bore will be better. With LS1 disc's all around I went with a 7/8" bore M/C. The 1" M/C that was one there didn't cut it.

RemoveBeforeFlight

I agree with you 100%. With dual rears you HAVE to run the 1-1/8" M/C because of the additional fluid volume needed. With around 700psi to the rears (soft pads) that is enough to hold 10psi with dual rears and doesn't require all that much pedal effort.

 

[sackracing]

I was thinking the larger bore was good with these cars

 

[RmvBfrFlght]

I was thinking the larger bore was good with these cars

A larger bore M/C is less line pressure (all else being equal). Manual brakes with a 1-1/8" bore M/C is not going to stop the car. With stock brakes the largest bore M/C I would use is 15/16".

For a comparison here are some numbers:

Stock '87 Regal with vacuum assist will generate nearly 1500 psi line pressure with 100 pounds of pressure on the stop pedal. That is with a 24 mm (0.945") bore M/C.

A manual set up with a 1-1/8" bore M/C will generate 614 psi of line pressure (same 100 #'s on pedal).

That is less then half the line pressure for less then half the stopping capability.

A 11/15" bore M/C will produce 884 psi of line pressure.

A 7/8" bore M/C will produce 1015 psi of line pressure.

Another part of the manual brake equation is pads with a higher level of friction. Off the shelf EE pads aren't going to cut it. The other is a little more expensive, but it is larger diameter rotors.

RemoveBeforeFlight

 

[sackracing]

I have the old school BGC large pads on the rear, and I am going to Aerospace discs on the front.

 

[Scott Atk]

Don't forget pedal ratio. As you move the pivot point up force is increased and pedal travel is decreased.

 

[RmvBfrFlght]

I have the old school BGC large pads on the rear, and I am going to Aerospace discs on the front.

OK, since the front brakes are being changed from stock, you need to look at the disc diameter and the front caliper piston area of the Aerospace brakes.

The piston area is important, which is why I went with a 7/8" bore M/C versus a 15/16" bore M/C. The LS1 front caliper has less piston area then the stock '87 G-body calipers. So they require a higher line pressure to provide the same clamping force.

However, at the same time the LS1 rotors are larger in diameter which provides a longer lever. This makes up for some of the loss in piston area.

The formulas for calculating brake force are on various web sites. Toss a simple spread sheet together and see what is required.

If you provide the disk diameters, caliper bore sizes and such I can run it for you.

RemoveBeforeFlight

 

[sackracing]

So would it be on the safe side to go with the 7/8 bore, regardless of the brake combination?

 

[RmvBfrFlght]

So would it be on the safe side to go with the 7/8 bore, regardless of the brake combination?

No, need to take into account the caliper piston area and disc diameter.

RemoveBeforeFlight

 

[sackracing]

the aerospace front calipers are 4 piston, with 10 1/4 diameter rotors.

 

[Louie L.]

If you saved the PM pedal, that has a decent ratio for manual brakes.

x2. This is what I have on my car with no issues. Stopped the car at Reynolds from 177 mph with no chute.

 

[sackracing]

x2. This is what I have on my car with no issues. Stopped the car at Reynolds from 177 mph with no chute.

What is the rest of your setup?

 

[87GNSteve]

My friend and I did the conversion to manual on my car; used the vacuum brake pedal. (attempted to use the power master pedal but it just wasn't worth it.) Master was a new 7/8 bore Raybestos.
For the pedal to MC rod, the one in the vacuum looked like it would be the right length. So after destroying the end of the vacuum booster.... which I didn't mind I was committed to getting this done(and I have 2 spare boosters) we test fitted the rod and it was perfect!
It just takes some doing of getting that thing out. But if you were to get a booster off at the junkyard (or one you don't need) you could easily crush the plastic end and pull/pry using channel locks and a screw driver. You need a dremel cutoff tool to take the crimped on plunger off but it was pretty easy.
For the grade 8 bolt at the top of the pedal, I bolted it in and retained the rod by drilling a hole though the bolt using the factory cotter pin set up; that was fun I only broke one drill bit doing that.
Otherwise they stop just as good as the vacuum brakes and I still know I have some air in the system I bled them twice but it was getting dark and I was getting hungry. I'm not too concerned since I will be putting the stainless steel braided lines on and will be breaking open the connections again. But they do stop good in a "panic" and I can hold a few pounds of boost as is.

 

[sackracing]

Called PA racing and got on the same page with them on my setup, they are sending me a smaller master cylinder and I will order my Aerospace brakes this week. But it will be at least 4 or 5 months before the car is running.