Am I expecting too much?

[67gtonut]

67 GTO manual brakes

Stock style Single piston calipers up front
Stock rear drums
Wilwood 7/8" tandem master
Adjustable prop valve

Should I be able to lock up the wheels under heaving braking?

Just installed the Wilwood master.... bled the crap out of it... took a bit, some air was trapped in the master. I have a decent pedal, little mushy, but not bad. I cant get front or rear brakes to lock...... Really never could.....

Am I heading for at least a small 7 or 8" booster to get enough clamping force to lock wheels? OR am I missing something in this manual set-up?

Thanks

Troy

[SLO_Z28]

What condition are the stock drums and rotors/pads in, and what pads are you using? I can easily lock up my tires, or the tires of almost any car, with the stock setup.

[BonzoHansen]

what kind/size of tires?

[67gtonut]

Raybestos pads and Rotors.... all with less them 1000 miles on em.....

235/60's on front
245/60's on rear
15" rims.....

[Apogee]

Considering the factory manual disc/drum setup used a 1" bore master cylinder and could typically achieve lockup when functioning properly, I would say you have an issue somewhere. The 7/8" bore MC should be providing 25% more caliper pressure as the 1" bore unit did for the same input force at the pedal, albeit with 25% more pedal travel as well. Your OE front calipers have a lot more piston area than a C5/C6 caliper (6.7 sq-in versus 4.0 sq-in), and we can typically achieve lockup with reasonably moderate pedal effort depending on the pad compound.

When you bench bleed the master cylinder, did you plug both outlet ports and confirm that the piston hydraulically locked when you attempted to stroke it? If not, then it's possilbe you still have air in your MC which could very easily be causing the symptoms you're describing. Also, you're running a disc/disc master cylinder, so you should probably be running a 10 psi residual pressure valve in the rear circuit (if you're not) to maintain pressure on the rear wheel cylinder seals so they don't leak fluid out or air into the system.

Tobin
KORE3

[67gtonut]

Thanks for the replies......

I did bench bleed the master the best I could...... I think I need to bleed the system even further.....

I bought speed bleeders for the rear and it did seem to get better. I have speed bleeders for the front now..... I will install them and go at it, and see if I can get it better.

I am not running the residual valve in the rear...... too many conflicting opinions if it is actually needed..... not ruling it out.....

[67gtonut]

On a possible related note...... I noticed when I was bleeding the rear yesterday, I did have to add small amounts of fluid to the front res. of the master, not alot, but I did notice the fluid level would drop. I did not expect that, and do not know if this is normal.

I have been over the lines.... no leaks anywhere.

Pedal does not sink when system is closed up...... just a lousy pedal in my opinion.

[Apogee]

The Wilwood master cylinder has a hole through the divider between the reservoirs about 2/3 of the way up from the bottom, so you will see some fluid crossover when the level is above that point. Is it possible that's where your fluid is going?



I would expect a very low, somewhat squishy pedal with your brake setup.

Tobin
KORE3

[67gtonut]

The Wilwood master cylinder has a hole through the divider between the reservoirs about 2/3 of the way up from the bottom, so you will see some fluid crossover when the level is above that point. Is it possible that's where your fluid is going?

I would expect a very low, somewhat squishy pedal with your brake setup.

Tobin
KORE3

Whew..... ok, that makes me feel better on that point....

OK.... what do I need to change to eliminate the squishy pedal?

Tobin.... I really appeciate your input.....

[Apogee]

Happy to help in any way I can. Assuming everything is bleed within an inch of its life and there is no air in the system, the best way to firm up the pedal feel and reduce pedal travel would be to move to a larger bore master cylinder. The OE manual master cylinder for your application was a 1" bore unit, although I think you may be happier splitting the difference with a 15/16" bore unit, a 12% step versus ~25%.

If you go to a larger bore MC and like the pedal feel, but still can't achieve lockup with a reasonable pedal effort, the next step would be to look at your pads. Raybestos offers multiple compounds ranging from low CoF/noise/rotor wear ceramic options up to fleet-duty semi-metallics in their OE replacement options, so I don't really know what you're running now. Low brake torques can be a symptom of low CoF pads, glazed pads and/or improperly bedded pad/rotor combinations among other things, so pads can and will make a big difference in the performance of the system.

Tobin
KORE3

[67gtonut]

Thanks Tobin,

I also just got off the phone with Wilwood.... That guy does agree with you as far as the 10lb residual valve would help greatly.... and he does also agree that the 7/8" bore wont ever give me a rock hard pedal..... it should give enough clamping force to lock the wheels.....

So...... it looks like a 10lb residual valve is in my future..... also thinking of picking up one of their adjustable prop valves...... since I have to open up the system again... ugh.....

it never ends.....

[67gtonut]

OK..... installed Speedbleeders in front...... bled the crap outa them..... Pedal does feel better. But I am not gonna mess around.... I ordered the 10lb Residual valve, so I am not putting the wheels back on until its installed..... Oh how I loath cutting and double flaring brake lines....... lol

[67gtonut]

Well, I installed the 10lb residual valve and just for the heck of it, swapped out my generic adjutable prop valve to a Wilwood one.....

Pedal is nice...... no complaints there....

But still cant lock the wheels..... new rear shoes and drums are still breaking in..... But cant lock the fronts..... Rayobestos Ceramic pads with a generic slotted/drilled rotor.....

Think I am down to a "friction" issue?

[Apogee]

Troy, pads can a will make a huge difference, so it's definitely a possibility. If the pads you have now are "EE" rated pads with a CoF in the middle of the .25-.35 range at .3, then by changing to a "FF" pad in the middle of it's range (.35-.45) at .4 will give you a 33% increase in brake torque without any other changes.

[67gtonut]

So......

EBC Yellow's

or

Hawk HPS's

any other suggestions....?

[RSX302]

I've dealt with the "not being able to lock up" issue a few years back. Ultimately found that manual brakes do not work well on heavy street cars (>3500lbs) unless you do one or more of the following:
Disclaimer; I also have big sticky Toyo’s on a 3800lb car. Lighter car, set-up and smaller less sticky tires will lock-up sooner.

Yes pad Cf is a huge factor but...if the extra pad Cf doen't do it for ya here is more info. Ceramic pads are only for show because of no dusting. Not too good at stopping.

*Higher Caliper pressure (1000psi min..1500+ ideal based on 4sq in caliper) required for pad Cf below .4 (street pads)
I used Willwood’s BP30's (.5-.6Cf) on 13" rotors, SL6's and 3/4 master. That locked them up! Stopped the car perfect, (to my liking) but at the expense of major dusting, squealing and rotor degradation. Needless to say I removed them. Oh yea. pad knockback started on the rear rotors with the Ford 9” due to the small master. Now I'm using power assist with large 1 3/16 master and Wilwood BP10’s. (.4Cf) Will put you through the glass..and lock up if I so choose.

*Increase Pedal Ratio Currently running 4:1 power assist and have run up to 6:1 and 8:1 for the previous manual system to try and get more caliper pressure.

*Larger Diameter Rotors (i.e. 13 to 14" nets approx +10% rotor braking torque)

*Power brake assist Just a hunch, but if you don’t like the higher pad Cf’s to get your desired braking, you’ll be switching over.

Good Web Calc:
http://www.jakelatham.com/radical/in...culators.shtml

[67gtonut]

Thanks for the reply....

Most of that I admit is over my head.....

Need to learn more about the pedal ratio thing.... But yeah, if better pads dont do it..... It may be time for at least a small booster.

[a67]

Be careful with the pedal ratio, too high and there isn't enough stroke on the M/C.

As for pads, I was going to mention the StopTech Street Performance brake pads. Have them on two cars, the one with the vacuum booster and y-body standard C4 brakes they are great. Much better then the PBR pads that were on there.

On the car with the manual brakes and a 7/8" M/C, these pads are OK. Stop the car they do, but not quite good enough for me.

I am considering EBC Blue pads and the CarboTech AX6 pads. Although friction wise, the AX6 is closer to the EBC yellows.

Bob.

[Skip Fix]

FWIW for comparison I looked at some old Bendix pads I have and the more street/organic are FFs and the semi metallics(rotor eating suckers) are EEs.

I thought '67 had a 4 piston front caliper on their disc systems,went to single in 68?

[RSX302]

Be careful with the pedal ratio, too high and there isn't enough stroke on the M/C.

Bob.

Bore size to caliper volume will determine overall pedal stroke. Pedal ratio will add a little more pedal stroke just because you will be able to push the M/C harder with the same leg input. If you have a nice solid pedal on a 7/8 master with a 5:1 pr. Then going to 6:1 or 7:1 will be minimal stroke difference.