Saturday, January 16, 2016

Brake Troubleshooting for Tri Five Chevy/ Disc Retrofit

Brake Troubleshooting
for a Tri-Five Chevy
   -Alan Arnell

Not long ago I wrote a post “That Nagging Hole.” LINK Within that article I boasted, “If you have 18” of manifold vacuum and the system has been pressure bled, there are not too many things left to cause bad brakes.”  Call me a pessimist or a realist, nonetheless I am having trouble with that hot sports opinion.  

The name I have given to my ‘57 Chevy model 150 2-door sedan Chevy Classic is The Hell Bitch.  I stole the name from Larry McMurtry's book and TV-movie “Lonesome Dove.”  Hell Bitch was the name of Woodrow F. Call’s horse because of her contrary nature.  I love my car, however sometimes during a emotional rage, I think she hates me.  I just know if there can be a problem with any work I complete on The Hell Bitch there will be a problem.  I better have a few tricks up my sleeve, before I boldly go forth with my disc brake upgrade project.

With that anchor around my neck, I have been brooding about what could go wrong with my disk brake retrofit/upgrade project that I have not looked up or thought of doing to my project.   It is not just my sometime glass half empty attitude that makes me feel this way.   I have been told by friends and I have read about all the troubles my fellow Tri-Five Classic Chevy lovers have had with new disk brakes. For example: Let us say, I have an imaginary friend who had just completed his new brakes upgrade.  During his test drive, the brakes worked, maybe, better than the old drums, but his car does not have the stopping on a dime performance he was hoping for with his ‘56 Belair 2-door hardtop.

He has a 350 CI-SBC with a TH350 transmission.  During his retrofit he added all new lines, hoses and a vacuum line.  The new setup has front discs with single piston calipers and a new power booster and master cylinder.  The rear brakes are drums but they had been given new shoes and springs.  He has 18 pounds of vacuum to the power booster.  Like I said, the brakes work but work about as well as an enlarged prostate patient trying to take a...well you know.

To solve my imaginary friend’s brake delima, I went to the books to research remedies.

The different automotive writings I search through state you should start with a brake pressure gauge
to check pressure at each caliper or drums and at each master-cylinder outlet port.  Be sure that you
have deactivated or bypassed the proportioning valve.  Under testing at full hydraulic pressure a correct pressure should range between 900 and 1,000 psi with manual brakes and 14,000 plus psi with a power-assisted brakes.  If you get a pressure less than those amounts at one or more of the wheels and at the master cylinder you have a problem with the in the lines or at the calipers and/or drums.  

Say that there is high pressure everywhere, yet the brakes do not lock up, then there are even bigger problems, because that tells that you have a master cylinder, calipers and drums that are mismatched or incompatible.  

If there is low pressure readings everywhere you may be looking at a bad master cylinder, power booster and/or brake pedal.  However, if you know for certain that the system has been bled correctly and you have a firm pedal with low pressure readings, that generally rules out a defective master cylinder.  This is because the cylinder is not bypassing internally.

Remember Occam’s Razor- “The simplest answer is usually correct.”

At this point, I would suggest checking to see if you have at least 14 pounds of engine vacuum, no vacuum leaks, and that the brakes have been bled correctly.  Just saying!

To check to see if the booster is OK you can use a command handheld vacuum-pump.  After extreme effort, you can make a vacuum in the booste with the hand pump.  When you have pumped out
18 inches of vacuum that reading should stay steady. If the pressure drops you have a defective booster.

Say you have a steady 18 inches of vacuum and the knowledge that your booster is OK.  The next thing to check is the brake pedal and how it lines up and operates the booster and master cylinder.

All brake pedals are really  fancy levers that increases a braking system’s force.

“Give me a lever long enough …  and I shall move the world.” - Archimedes

A manual brake system usually has a 6:1 pedal ratio with a ⅞ or 1” bore master cylinder.  A power system on the other hand only needs a 4:1 ratio using a 1 ⅛ “ bore master cylinder.  With that in mind you must check to see if your pedal has enough travel to make a long enough stroke in the master cylinder and power booster to move the correct amounts of brake fluid to engage the brakes.  Double check to see if your setup is producing a full stroke  

Even if there is enough engine vacuum, if you don’t supply the maximum stroke to the master cylinder you are not getting the highest level of braking power.  It is like your if you chihuahua named Tinkerbell got behind the brake pedal while you are trying to stop.  The push rod must be given its full measure of travel to make your brakes work to their highest level.

Pedal advantage is determined by measuring the length of the pedal arm from the push pedal to the pivot point.  To measure this distance correctly you have to measure the distance straight down from the topo pivot while using a level to mark the the arc of the push pedal.  By doing so,
you are making a triangle.  The pedal arm is the hypotenuse of the triangle, the horizontal swing of the push pedal is the short leg of the triangle and the vertical distance from the swing to the pivot is the long leg of the triangle.

You want the long leg of the triangle measurement to be divided by distance between the pivot and the master-cylinder pushrod hole. Again use the triangle method to determine the distance.  


During the era of Muscle Cars, Chevy brake pedals came from the factory with an upper and a lower master-cylinder pushrod hole. The top hole was used for manual brakes and the bottom hole was used for power brakes.  Remember that power brakes require less ratio than manual brakes.

Another problem to consider is that some smaller boosters may not allow enough stroke with big production brake systems.  If you do not have the room for a larger booster a smaller-bore master cylinder could be used to fix the problem.  (I don’t even want to think about the troubles you would have to overcome to make that work?)

The last things to consider.  Your brake line should be hard lines up to the point they connect to the brake calipers or to the "T" to the rear axle hard lines.  The rubber hoses from that point to the brake cylinder or calipers should be a rubber teflon-core hose.  Never use a rubber-core hose you would use for fuel or oil lines.  

Another alternative is to use teflon-core stainless hose.  This hose is the best to use for more powerful braking systems because they will not expand as easily under high braking pressure or heat.  That expansion of the line will degrade the amount of braking power available and usually at the worst time.

I hope you and I do not have to use this information.  But knowledge is power, or so they say.

Links to more Power Brake Posts:

Tri-Five Late Model Power Booster Conversion Part 1

Power Booster Conversion Tri-Five Late Model Part 2
Power to the Front
Move to a Dual Purpose MC and Disk Brakes
Front Disc Brakes for a Tri-Five Classic Chevy

Power Booster Conversion Tri-Five Late Model Part 3
Tri-Five Classic Chevy
Proportioning Valve, Dual Master Cylinder and Brake Lines

Brake Pedal Clevis Relocation After A Power Assisted Disc Brake Upgrade on a Tri Five Chevy.

Front Wheel Alignment

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