After 27 years I finally have my Brakes working properly.

[JON1D]

Interesting read and the solution to your problem. Its sometimes a simple fix to a head ack problem and I guess a few people having the same issue of poor brake pedal pressure.
I know when I did my brake caliper strip down fitting new pistons and rubber seals one thing was keeping everything as clean as possible and lots of lube on the pistons . When the calipers are zinc dipped I guess they cause some friction on the sideways movement of the pads on the bottom face so its worth removing this plating where movement is required.

[Eggbert]

Finally, I solved my braking issue I've had since building the car 27 years ago. I made up this post from bits and pieces of other posts I did at the time in another forum.

To start, I did a lot of brake line pressure testing and the pressure should have stopped the car close to 0.9 Gs according to my calculations. I forgot about my G-Tech meter, so I went for a test drive. All I could achieve was 0.64 Gs, which works out to 187 ft to stop from 60 mph.

My Cobra OEM states they got 135 feet from 60 mph. That works out to about 0.89 Gs based on 2800 lbs for car, driver and fuel.

I decided to bleed the brakes once again and this time I removed the pads and re-surfaced them with sandpaper. I also scuffed up the rotors a little bit. A good improvement after bedding in the pads again. The nose of the car would now squat down significantly under heavy braking. The 0.64 G's I previously had mentioned was a one time thing. The repeatable number was 0.61 G's. Now I was getting 0.67 G's and it is repeatable.

Still curious, I mounted a tape measure under the car to measure the front master cylinder stroke and found that it was bottoming out. I bought a Tilton rebuild kit and changed it out. The old piston and seal showed no signs of wear and the cylinder bore was immaculate. No difference after doing this.

Thinking something was yielding anyway, I installed SS Teflon lines. No difference after that. So it's air. So I bled the brakes both with the vacuum method and reverse bleeding. The last time, I removed the calipers and held them up so the flex line was vertical and tapped the caliper to loosen any air bubbles. The net result was 550 psi where before it was a little over 500, maybe even 540. Hard to be accurate with such a small gauge.

When I look back, hindsight tells me I made too much of this, however there were a lot of factors leading to confusion. First of all, my measured brake line pressure of 550 psi should have yielded over 0.8 Gs, but I only got 0.67. The fact that the front MC would slowly drop the pedal was not very noticeable and I didn't realize this was happening for a long time.

I went for another a test run and it didn't go as well as I hoped. 0.7 G's was it and I was hoping for 0.8+. I started out cautiously trying to brake hard. A little harder each time. With the 0.7 G number, the right front wheel pulled to the right so there must be an issue with the left front caliper or pads. I stopped testing after that.

I suspected if I removed the caliper and extend and retract the piston a bit (like I did to the other side), re-sand the pads and re-lube the slider pins it should fix it.

I have to admit, going from 0.61 Gs when I started testing to 0.7 Gs seems like a huge difference when braking hard. Also, now the front end drops so much, the G-Meter goes out of calibration, so I might be getting a little more than 0.7. You calibrate it while on level ground and any variance from that drops the measurement a small amount.


I did a test drive. 0.72 Gs and I know I didn't press the pedal as hard as I could have, but close. The car did not pull to either side.

Just as I've run out of ideas, a new thought just occurred to me. I may have screwed up. Although the D52 calipers have slide pins, some of the force on the caliper from the spinning rotor appears to be taken up by the caliper in contact with the knuckle. This is an area where sliding occurs when pushing the pedal. It never occurred to me to grease that area. I then tried  some brake grease.

Well.... success!!! I managed 8.6 Gs on the next test run. I think I might still have a bit left but cut the testing short as not only was I happy with the 0.86, but it was only 4 degC (39degF) outside.

Now previously all I ever did with the silicone brake grease was lube the guide pins which is typically almost all you do with most disc caliper systems. The other spot for grease in the little end ears on the pads. But these D52 pads don't have the tabs. The one against the piston does move of course, so I greased the edges of the back-plates where they rubbed on the caliper as well as re-lubing the little spacers where the guide pins go through and also re-lubed the guide pins. Didn't lube anything on the outboard pad as it doesn't move in the caliper, but did add a little lube around the guide pin holes in the pad and re-did the hole in the caliper where the guide pin end goes through.

However, as can be seen below is a picture (not my car, but a pic I found on the net) there are abutment pints on the knuckles. The caliper slides on these in order to move the inboard pad against the rotor. Here's where I added lube that never had any before:



Now these D52 calipers have been used for well over 20 years in many GM vehicles, but nowhere on the net could I find anything about greasing the abutments. It is interesting to note that a picture I found of the Wilwood equivalent doesn't mention it either but clearly shows where they didn't paint the caliper in order for caliper to slide on the abutments.



Of course those abutment areas are a great place for the grease to pick up brake dust dirt so I'm not sure if they were meant to be greased. However I suspect that those vehicles that used them also had a booster so extra pressure to overcome the friction was available.

My next step was to try the Yellowstuff D52 pads I bought. I already have them in the rear. I'm going to wait until Spring as they need to be run a few hundred miles to wear away the coating EBC puts on them that's supped to clean the disc of old pad material.

I may also try the Wilwood calipers, but they don't recommend them with DOT 5. Of course Tilton doesn't recommend their MC with it either, but I found no evidence of seal problems with the old MC I took apart. 25 years of being immersed in DOT5.

To summarize, all the things I tried helped, but were not the major issue. Larger calipers, different pads, new master cylinders, testing line pressure, calculating G force from brakes starting with so many pounds of force on the pedal and who knows what else (I can't remember any more). The key to the whole thing was to grease the abutment points.