QuestionQUESTION: I just rebuilt the entire brake system on a 76 spitfire. New rear cylinders, caliper rebuild kits, new hoses and a rebuild kit for the MCylinder. My brakes are spongy, even after numerous bleeds. My pedal goes 50% to the floor, and then firms up. If I pump them a bit they get firmer. It doesn't seem to be air in the lines, and my shoes are adjusted out, so its not due to the shoes having to engage.
Even after the brakes are pumped up the braking is not very positive. I'm wondering if I fudged up the MC rebuild, sort of vague instructions.
ANSWER: Hi Jeff,
It may not be anything you did wrong as there are several factors involved with new brakes.
First is that new shoes often don't exactly fit the drum so some flexing of the shoes can add to a soft pedal. This can be checked as a cause or part of a cause by just adjusting the rear brakes up tight as to lock up the rear brakes. Then check the pedal and see if it improved. I use to go to the next better thing and take a bleeder valve that fits the master cylinder line hole and put in place of the rear line, this then eliminates the rear completely. Don't forget to readjust the rear brakes correctly afterward.
The second thing and just as common is the method that caliper manufactures use to retract a piston away from a pad when the brakes are released. The caliper piston seals are square and the piston does not slide easily in the seal. When you step on the brakes you force fluid against the piston and the piston moves toward the disk. If the distance is great (as when you rebuild a caliper) it first flexes the square seal and then slides toward the disk until it reaches the disk. When you release the brake pedal, the pressure in the system is released back into the reservoir and there is no pressure on the piston. However the caliper piston seal was flexed into a trapezoid shape and now it returns to square which retracts the piston. This is exaggerated on a new rebuild.
To test if this is the cause jack the front up and remove all the new pads and replace them with the old ones (or if you have already disposed of them just temporarily put anything just a little thinner than the new pads in (anything will do as you are not going to drive it)
When you have the thinner pads or what ever in place pump up the brakes a little and then remove the thin pads and pry each piston back into it's bore but ONLY enough to squeeze the new pad in. If you force the piston in far enough to easily slip the new pad in, you have to redo the process. The end result should be that you had to force the new pads in and the wheel will be hard to turn by hand. The seals are now flexed in the wrong direction so as to hold pressure on the pads. Do both wheels and when finished check the brake pedal. If it is now correct, you are not done but have found the problem. If the pedal is now correct let the car down and drive it in circles right and left hard in a parking lot and if you don't have a large parking lot drive the car in 1st gear making hard right and left "S" turns for at least half a block then jack the car up and check that the wheels are not hard to turn by hand. DO NOT JUST DRIVE THE CAR BEFORE DOING THIS! It will burn up your new pads and probably destroy the new seals. The hard turns flexes the wheel bearings etc so as to push the caliper pistons in just a little. NOTE! if your wheel bearings are loose this can be a cause in itself.
All of this positions the new seal in a neutral position close to the brake pad & disk.
Let me know.
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QUESTION: Ok, I went through this last night, and rebled the brakes again, the sponginess is gone, and I havn't taken for a drive yet to see how the stopping is. The brake pedal travel still seems like it goes way too far to the floor before the brakes engage. I was watching the plunger movement at the master cylinder while one of my boys pumped the pedal. piston travel is about 1.5 inches then the pedal stops. After opening the bleed nipple the plunger only travels another inch or so before it bottoms out. Should there be so much pretravel in the MCylinder? When I rebuilt the cylinder I think got the seals in right, but there were no instructions,so I bought another seal kit from a higher class firm that actually provided instructions, I tore the cylinder back down and it looks right, but I'm curious if fluid leaking between circuits within the cylinder could be causing the grief? Then theres the PDWA, if this is not positioned correctly would this cause a problem? Idiot light is out and this idiot has been too lazy to replace the bulb.
ANSWER: Jeff, if you don't want to do what I suggest that is fine. My advice is free so nothing lost. When I received a car with your problems I had to do what I told you to do, if you have a better way to fix them, great.
The PDWA is nothing more than a switch activator to warn you that there is a difference in pressure between the two brake circuits and it has no other function. Quit looking for things to replace, fix the brakes.
Howard
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QUESTION: I did what you told me to do the other night, I just hadn't taken it for a ride. I made sure the rears were set tight, and I pulled out the antisqueal plates on the pads and pumped the brakes, reinstalled the plates, they are about.015" thick, and the front wheels now turn a little hard. I took for a ride last night and the braking still isn't very positive, the sponginess in the pedal is gone, but you really have to stand on the pedal to stop the car. Thats why I was asking about the master cylinder, if it wasn't functioning properly could it cause these problems.What is strange is if you push the pedal down and try and try an turn the wheel by hand, the brakes are obviously working, you can't turn the wheel.
AnswerWhen you have taken the soft feel out of the pedal that is the end of any hydraulic problem. If you have good pedal and it will not slow the car as it should then that is either you have changed the bore size of the master cylinder to a larger one or the disks are not smooth or the brake pad material is wrong for the car.
Do this,
Find a hard dirt road and go about 20 MPH and apply the foot brake steadily until you hear a wheel slide and back the car up and look at the skid marks. You should see that both front wheels slid and the rear didn't. Then do it again and this time lock the brakes hard and look at the skid marks again, you should see all four skid marks. Now take it to a back street and go about 10 or 15 MPH and try to lock up the brakes. You should be able to make the front brakes at least make black marks. If you do and the dirt road tests were ok, your brakes are working about normal and as the new shoes and pads seat in the brakes will get better (easier to skid)
Howard
