Symptoms of having the wrong sized Master Cylinder?

[jlwdvm]

What would be the symptoms of having a master cylinder that is the wrong size? I have a manual Wilwood 7/8" (rebuilt by Wilwood) on my project with Aerolites front and rear, all new copper lines, stainless flex lines, bled...driven...bled...driven, vacuum bled, reverse bled, pedal pump bled, pads bedded, etc. It just seems like the brakes don't wake up unless I really slam on them with my foot...almost like I have to think ahead about stopping when I am driving around town. I have a '15 Z/28 and a '17GT350R and the slightest touch of the pedal gets an immediate response (granted they are power brake cars). I'm just not sure what a manual brake car is supposed to feel like. I can get the brakes to lock up if I really slam on them. Thoughts?

[Oleyellar]

Not enough info on the car to know for sure, but, some ideas....proportioning valve setting, or is there one? Do you have one with a switch that could have shifted? Do you know for sure the front brakes are functioning? I had a situation where the master cylinder was not allowing full pressure to the front calipers. There fronts would show some marks on the rotors and would clamp when turned by hand, but were of no use while driving.....Is your Willwood master one of the recalled ones? Are the front calipers sized properly for the weight of the car?

[Vegas69]

Modern power brakes vs. manual brakes are a night and day difference in feel. You should have a fairly firm pedal that is easy to modulate under heavy braking.

[jlwdvm]

Brake calculations done by Ron Sutton and complete package purchased from RSRT: Front Aerolite 6 Brake Kit, Rear Superlite 4 Brake Kit,Tandem M/C w/Built In Reservoir, proportioning valve for a 3500# '69 Firebird with Toyo R888 275 and 315 tires. Last summer I had pressure gauges on the calipers and they speced out with good pressure after the rebuilt/recalled master was put back on.

[dhutton]

I would try a more aggressive pad compound. Easiest thing to try at this point.

Don

[jlwdvm]

Looks like I have BP-10 pads in at the moment.

[RidiCat69]

Originally Posted by Oleyellar

....proportioning valve setting, or is there one? Do you have one with a switch that could have shifted? ...

This is very real. If the prop valve/combination valve has a safety switch and you bleed the system with no pressure on one side, that side can close off. The combination block thinks there's a leak and is trying to save one of the circuits. If you do have one like this, the plug in the middle of the valve can be removed and you can visually inspect to see if its tripped. A tool is actually sold to hold this pin in place while bleeding an empty system.

You said you had good pressure when you put the gauges on the calipers. Remember what that pressure was? Was it equal on front and rear?

What is the pedal ratio? If you're coming from a manual drum or a power disc the input pressure wouldn't need to be as high as the manual disc requires.

Here's my favorite one that I've see converting from a manual drum car to manual disc. Firewall deflection. It doesn't matter how hard you push on the pedal if the firewall is absorbing all the pressure.

Binding push rod into the master can be something to look for too, or how about the bushing between the pedal lug and the pushrod, or the bushings for the pedal itself?

[jlwdvm]

Master is in the manual pedal hole and the bushings for the pedal and clutch are new. I have an adjustable Wilwood proportioning valve and I can get fronts and/or rears to lock up if the brakes are warm and I really slam on it.

I can't remember the exact numbers we were seeing, but they were fine for the system.

I guess my feeling with the system at this time is that when I am driving around town I seem to have to think ahead to stop. A panic stop in traffic might be a little iffy.

[Oleyellar]

Looks to me like you have everything working properly for a manual brake setup. Time to start doing squats, and get those thighs toned up!

[Vegas69]

Some don't like when I say it, but manual brakes on the street aren't ideal. They are made for the track.