hard brake problem

[Guest 77turboZ]

Good evening everyone! I need you guys help in trying to figuere out what is wrong w/ my brakes. Perhaps someone else has gone through this frustrating situation and can lend me a hand.

 

I can't figuere out what has happened to my brakes. I have checked the check valve, bled the brakes and still brakes are hard and will not stop the car. Last winter I purchased the ross c. 240sx rear brakes kit and this winter the JSK Wilwood front brake kit using the 12.19 rotor. The calipers are the Forged Superlite 4 piston w/ the 1.12" pistons. My master cyl. is 15/16. from a 280zx and my master vac is the stock 280Z for my 77. does anyone have any advise as to what I may be overlooking or have wrong in this set-up. Any help would be greatly appreciated.

 

Thank you in advance.

[Guest Drewz]

How much pedal travel do you have? Are they consistently hard as a rock at teh same point of depression of the pedal? Is the booster hoked up and receiving FULL vacum with an inline check valve? the check valve has on occasion failed and blocked the line preventing vacum assist. rare but have encountered it. Open the bleeders and see if fluid comes out when the pedal is pressed at all 4 corners. has the car sat for some time? Were the rear calipers new or used or remans? Just a few questions and starting points.

[blueovalz]

Can we assume the problems started when you put the Wilwoods on the front? The problem I believe is with the Wilwoods, and their small piston diameter. In effect you've got less than 2 inches square of piston area (using one side of the caliper as the reference here) in comparison to what came from the factory. I'm not sure but I believe the OEM calipers had a 2" or larger diameter piston (again, one side only), which means you've increased pedal pressure by at least a minumum of 70% (or decreased caliper pressure by the same amount). You've gone from more than 3" of piston area to less than 2" on your calipers. Now, to add insult to injury, you've also put a larger MC on the car as well, which exasperates the situation. This factors in another 11% increase (going from memory on this number) verses the OEM 7/8" MC.

I went from the OEM MC to the 15/16" MC only after going to the larger 2.375" piston size calipers (again, as a reference using one side, which was equivalent to just over 4 inches square of piston area), which then just about neutralized the pedal softness of going to a larger caliper piston (the caliper piston size increase was about 12% in my case if I remember correctly). In your case you went the other way; to a larger MC and a smaller caliper piston (total piston area), which now decreases your leverage of clamping force on the rotor by quite a bit (in fact by a lot). I would have gone to a caliper with 1.5" pistons (which would have given you about 3.5 inches square of piston area on one side of the caliper), and left the OEM size MC in place, which would have been a much better, if not a great match.

[Guest 77turboZ]

I have just about zero pedal travel. The problem started towards the end of the year. the rear calipers were reman. but they were working great up until just before i put the caar away for the winter. blueovalz, if I understood what you were trying to say correctly, than I should go w/ a smaller M/C? if so , what should I go to? I really what to drive my car and would like to get this resolved. Thanks again.

[waynekarnes]

when you bleed the brakes, do you have pedal travel ? any rough spots where you feel hesitation or drag, push slowly ... you may have a bad master.

 

if you get fluid and easy pedal movement, then i'd look at sticky calipers or a bad pressure regulating valve at the rear of the car.

 

or as was suggested earlier, larger bore master, smaller bore caliper, you lowered your ability to create high pressure at the master and are unable to push fluid through a small hole.

 

imagine this ... think water ...

 

low pressure at the pump, restricted hole at the end of the hose, requiring a high pressure pump. only a trickle comes out and the pump slows down because it can't push the water, eventually it can't move at all, as it's not meant to move water at that pressure.

 

same thing with your master, it's meant to move a certain amount of fluid, through a certain size hole.

 

think leverage, and fluid dynamics.

 

but ... also check your power brake booster, is the hose connected, not collapsing, the check valve in place and working. is your engine producing enough vacuum to make the power booster work ? was a big problem back in the 70's with big cammed engines.

 

are the rear brakes locked against the rotors as if the parking brake were stuck on ?

 

car on jack stands, level ground, wheels off, engine off, have a friend watch all four calipers are they moving, sqeezing and releasing ???

 

if not a mismatch between the master and calipers, calipers are working properly, you used a vacuum gauge and did a leak down test on the booster, temporaily by pass the check valves in the rear, they might be stuck.

 

it sounds as if the calipers are not letting go. take a look at those rear pressure regulators, if all else checks out.

 

wayne.

[blueovalz]

This was just an observation. If indeed the brakes worked fine for a time after the Wilwoods were installed, then perhaps my comments are in error, but I feel strongly that these caliper's pistons are too small. Brake hydraulics are simply a fluid lever. Datsun designed a good working ratio with the hydraulics, and our modifications need to stay as close to this ratio as is practicable.

Again, I do not recall the piston diameter of the OEM calipers, but believe they were about 2.125". The OEM MC was .875". This creates a ratio of .1695 when comparing piston AREA between the MC and the Caliper piston(s), or the MC/SC ratio. To arrive back to this ratio, you really should be using calipers that have 1.5" pistons (4 piston type), which is almost perfect with the OEM MC size of 7/8".

Now on to your question about MC size. In order to maintain the OEM ratio with your Wilwood calipers (front brakes only in this study), you'd need a MC bore size of between 5/8" and 11/16". Your current ratio is about .304, which is about double my estimated OEM ratio. This means that the pistons at the caliper are moving toward the rotor at roughly twice the normal distance with each stroke of the brake pedal (again, based on the OEM sized MC and not your bigger one which makes this even worse). What this then "feels" like is a very firm pedal with much less stopping or clamping force at the rotor than the OEM feel would have. Much like using a too short a wrench in braking loose a nut. Not much leverage and a lot of resistance.

Unfortunately this effects the rear brakes as well. Your current set-up biases the rear brakes more than normal, and with that said, you may find you will need a proportioning valve that will limit the pressure to the rear calipers being they are more "correct" in their size compared to the OEM requirements.

[waynekarnes]

some one here and over at list240.org, suggested the magazine grassroots racing. man is that magazine ever chocked full of info.

 

suggest picking it up and reading the article on pressure regulators and proportioning valves. a very good read. written by a engineer that works for delphi.

 

 

wayne

[Guest 77turboZ]

the pedal travels fine when I am bleeding the system, no interruptions. I have the car in my garage and it moves quite well. The rear brakes seems to be what is stopping the car not well but they seem to be working.

I will check the regulator valves and see what happens. Would those regulator valves keep the front bakes from working? The brakes worked prior to the install of the Wilwoods although they began to feel a bit hard.

 

you guys have been super thank you for all you assistance.

[waynekarnes]

you should pick up that magazine, grassroots motorsports. that article on brake biasing is a worthwhile read.

 

the purpose of a brake pressure regulator is to prevent the rear wheels from locking in advance of the front.

 

the regulator, brake regulator, biases the percentage of braking to the rear versus what goes to the front.

 

it attempts to balance the braking. in other words, as the car stops, it wants to keep moving forward. inertia. the ass end of the car lifts slightly, makes the rear end lighter, the front presses down, makes the front end heavier.

 

your front brakes do the majority of your work to stop the car, so biasing puts more brake pressure to the front brakes, they start stopping the car before the rears do.

 

once the fronts have slowed the car, pressure is high enough ( from driver pushing harder on the pedal ), the pressure regulator in the rear is over come by pressure, the rear brakes now start working.

 

should the rears come on too soon, they cause the rear to slip and slide, remember, under braking, the rear is lighter than the front, the tires loose traction, rear slides, skips, driver gets excited.

 

that's the main purpose of a brake pressure regulator. sometimes, they also maintain a bit of residual pressure, on the wheel cylinders on drum brakes, so that the shoes don't have as far to travel.

 

the z car comes from the factory with the brake pressure regulators, set by engineers, to match the original specified bores of the rear wheel cylinders and the bore of the front calipers working in unison with the master cylinder.

 

for the most part, it will work fine when converting to 4 wheel disc brakes. it seems to maintain the proper balance. to test this ... in a safe area ...

 

get car up to 31 mph, in a straight line, brake hard. cause the tires to skid.

 

the skid marks for the front wheels should be longer than the marks from the rears.

 

there is no adjusment on the regulator and it is recommended, that it be replaced every 30k miles.

 

 

engine running, pull and plug the hose to the booster, any difference ? if so, then most likely not the booster. if no difference, engine running, check the hose for sucking ( vacuum ). if none ... find out why. if so, and you already checked the the calipers, ect. and they all appear to be correct. i'd suspect the master, the way the plunger goes into the master, the length of the rod into the master, or the booster.

 

if you had no problems before the swap, work your way backwards.

 

normally, a regulator will not cause the issues you are having, a bad one will most likey, either cause the rear brakes not to release ( once the pedal is pressed ), or not work at all, causing the rear of the car, to dance around under hard braking.

 

pull the front hoses off, bolt em to the original calipers, bleed em, stuff the old rotors in place. see if the pedal works now. if so ... it's the new calipers.

 

if no change ... leave the old stuff hooked up.

 

go to the rear ... hook up the rear hoses to the brake drum rear wheel cylinder. put a rope or string around the business end of the RWC so the pistons don't shoot out. bleed the rear.

 

 

if all is well and you have pedal travel, the issue is in the rear.

 

if not, check the regulator and the master.

 

but 1st, i'd check that rod that goes into the master, is it centered, did it fall free, is it too long, ect. then i'd check the booster . then the front calipers ... pull em off the rotors, slowly press the pedal ... if goes down ... you have the wrong pads, wrong rotors. if don't move ... then start swapping parts out ... .

 

too much work ? sorry ... i can't think of an easy way to do this with out standing there looking at it with my hands on it.

 

wayne

 

how's the space between the new calipers, pads and the new rotors ... is there any room for them to move ??? if no room to move, no pedal travel.