Low Brake Pedal on 65 with Power Brakes

[Elliott P.]

Re: Low Brake Pedal on 65 with Power Brakes

Hi James West, I am not quite sure what you are asking in the picture. My complaint is that when my brake pedal is fully depressed, the pedal is too close to the floorboard. What I see in your picture is that in the at-rest positions, your clutch pedal appears to be much further from the floorboard than the brake pedal. The brake pedal at-rest is determined by the hard-rubber return stop under dash, which looks about the same as mine. Your clutch pedal is probably adjusted for softer action/more travel. I have my clutch pedal adjusted for quick action/less travel, and then the clutch and brake pedals at-rest are lined up at about same distance from floorboard. I am not sure which way they came from factory, but it is an easy change made under the dash (I trust this could be a dealer service option before delivery as far as NCRS is concerned). I hope this answers your question.

[Elliott P.]

Re: Low Brake Pedal on 65 with Power Brakes

Thanks Tom, I will check that out. I can certainly see that excess lateral run-out would effectively position the brake pads further apart, thus requiring more brake pedal travel

[David H.]

Re: Low Brake Pedal on 65 with Power Brakes

My 1965 L79 with power brakes has a very low brake pedal, after all linkage adjustments and brake bleeding. I have been told that this is typical of these cars by several persons. Is it unique to the one-year-only master cylinder (first dual-cylinder), or anything else unique to this year? Even if it is correct, I do not like it. How do I fix it?

Elliott

1st potential problem: Do you know how (if) master cylinder was bench bled? If bleeding was done with MC on the car, then car's rear should have been elevated to level out the Master Cylinder in order to prevent trapped air. See notes and link below.

NOTES:

1. When bench bleeding the master cylinder while mounted on the vehicle, be sure the master cylinder is level. It may be necessary to raise the back end of the vehicle to get the master cylinder level.
2. If the instrument panel brake warning light is illuminated with the ignition switch on, depress brake pedal several times. If warning light does not go out, bleed system again.

https://www.performanceonline.com/blog/2009/02/master-cylinder-bench-bleeding/

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2nd potential problem: If MC bench bleeding was performed off the car, then there is a possibility of damage to MC piston seals. Some vendors recommend bleeding strokes no more than 1 inch deep in order to prevent seal damage. See note and link below.


Then, install the supply plugs into the primary and secondary brake line ports of the master cylinder and fill the reservoir halfway with fresh, clean brake fluid. Using a blunt tool, depress the piston using a short stroke of an inch or less and hold for 20 seconds. Make sure the stroke doesn't exceed one inch or else you could damage the seal. Keeping the piston depressed is the key to getting all the trapped air out of the quick take-up valve. It's important to wait at least 30 seconds between strokes, to allow the larger primary bore to refill the fluid. Repeat as necessary, until no air bubbles are visible and the piston travel is less than 1/8th of the original stroke.

https://www.buyautoparts.com/howto/h...r-cylinder.asp

Dave

[Edward D.]

Re: Low Brake Pedal on 65 with Power Brakes

Elliot, I think Duke has it nailed. I had the same problem with my 66 which uses essentially the same m/c as your 65. Found pitting in the bore of the front half of the m/c which resulted in leakage past the seals making travel of the piston much too far until braking action. A rebuild kit wasn't going to fix this so I sent it out to Lonestar Caliper in Texas to sleeve and rebuild. This fixed the problem. Pedal nice and high and both freeplay and distance from floor good.

[Elliott P.]

Re: Low Brake Pedal on 65 with Power Brakes

Thanks all. I think every nail must have been hit by now. Known givens: all calipers were replaced recently with sleeved and rebuilt calipers, with no leaks, and several attempts were made to bleed brakes, but procedure not known. Master-cylinder is not off the suspect list.

Proposed procedure:

1. Lift rear of car to level master-cylinder and remove two rear tires.
2. With rear tires off, check lateral runout of rear rotors, correct if needed as suggested above.
3. Pressure-bleed all four calipers per 6-step procedure I had included above, not overlooking outer bleed-vales on rear calipers.
4. May need to repeat bleeding several times. (and tap, tap, tap)
5. If no significant improvement, remove master-cylinder and check dimension of power-booster push-rod extension; if not in spec, have the power-booster rebuilt and replaced. While off, disassemble master-cylinder looking for non-sleeved bore, pitted cylinder bore, or damaged seals (hopefully finding something). Replace bench-bled, sleeved and rebuilt master-cylinder.
6. Repeat steps 3 and 4.
7. If no significant improvement, curse the engineers who designed these calipers. (I know it will do no good, but it feels better.)
8. And Wilwood makes bolt-in replacement calipers of superior design.

Any other suggestions?

[Duke W.]

Re: Low Brake Pedal on 65 with Power Brakes

I would suggest you first get all four tires off the ground. Have a helper apply firm pedal force and you try to rotate the tires. If either the fronts or rears rotate easily, that side of the m/c has failed, or the m/c was not properly bench bled. In my experience it is very difficult to bleed the system if a new m/c has not been properly bench bled.

Duke

[Elliott P.]

Re: Low Brake Pedal on 65 with Power Brakes

Thanks Duke, I will make that step zero.

[James G.]

Re: Low Brake Pedal on 65 with Power Brakes

Are you sure of the type of brake fluid? Silicon does not mix with DOT 4. Previous collectors may have used SILICON