Brake Help Please

40 · October 15, 2005, 11:58:33 PM

Did I mention before that I HATE brakes??I'm trying to finish up my 36 and drive it a bit before the snow flies and am having trouble getting a good pedal.It is an under floor,4 wheel disc set-up with a 7" single diaghram power booster.I have 16 to 18 inches of vaccum,the correct residual valves,have bench bled the master cylinder and bled the system 3 times...once manually and twice with a power bleeder...once from the m/c outward and once from the bleeders in.Have I ever told you guys how much I hate brakes :x With the motor off,I have a great feeling/firm pedal but the moment I start the car....The pedal softens and goes ALMOST to the floor.I have adjusted the push rod to the point that I have just a slight touch of drag on the brakes.When I engage the pedal,all of the calipers seem to be moving the pads etc correctly...I can stop the wheels but it just barely holds the car still at idle and the pedal is within a 1/4" of the floor.Have I said before how much I HATE brakes! I am not totally sure of the bore size...I think it's an inch and an 1/8.I am also not real sure of the manafacturer as this setup came with a project I purchased...it was new in the box but had no info with it.It is the typical Corvette style setup sold by ECI,Master Brake etc.I am not a big fan of single diaphram boosters...I always order a dual but I have seen this identical set up used many times with good results and am thinking I may have a defective booster or M/C? Any thoughts or help will be greatly appreciated....Man,I HATE brakes

enjenjo · October 16, 2005, 12:35:51 AM

Two possibilities

One, there is air in the system yet

Two, you have a bad residual check valve

Bruce Dorsi · October 16, 2005, 04:13:08 AM

Quote from: "40"With the motor off,I have a great feeling/firm pedal but the moment I start the car....The pedal softens and goes ALMOST to the floor.

If you can apply the brake with the engine off and get a hard pedal, keep your foot on the pedal while you start the car. (Not sure if this is what you did.) ....Does the pedal drop as far?

If you pump the brakes rapidly, does the pedal get hard again? ...If yes, I agree with Enjenjo.

Quote from: "40"I have adjusted the push rod to the point that I have just a slight touch of drag on the brakes.

If all the pads are in contact with the rotors with your foot off the brake, then there would be very little fluid moved by the m/c upon brake application, so the pedal should remain firm. ....I don't think a faulty rpv would apply here, but trapped air, or a bad booster may be at fault.

Which push rod are you adjusting? ...The rod between the pedal and the booster, or between the booster and the m/c?

Quote from: "40"When I engage the pedal,all of the calipers seem to be moving the pads etc correctly. ...I can stop the wheels but it just barely holds the car still at idle and the pedal is within a 1/4" of the floor.

Possibly not enough assist from the booster?

TOO MUCH travel of the brake pedal? ......Many cars with power brakes required less travel of the brake pedal, and reduced the pedal ratio by moving the push rod closer to the pivot.

Could it be possible that the rod from the booster to the m/c needs to be longer, and the rod between the pedal and the booster needs to be shorter? ....(I'm not sure about this, just thinking aloud!)

48ford · October 16, 2005, 07:26:59 AM

One time I got triped up on something like this and it turen out that I had the calipers on the wrong sides,It was on a mustang and it looked like I was bleading out of the top of calipers ,but WRONG,look close.
Russ

40 · October 17, 2005, 08:58:44 AM

Well...I had another go at the brakes last night.I replaced the residual valves with the ones from the 37 chassis I'm building and re-bled the entire system....same result :x I also removed the booster and varified that I had about 3/32" clearance on the rod between the booster and M/C.An old timer told me to push the plunger in on the booster,hold my thumb over the hole and see if the pressure bled off...If not,he says it's definately not the booster....I did this and it held so am thinking the booster is probably OK.In response to the questions above....I have a firm pedal,with my foot holding the brake,it softens and goes nearly to the floor when the car is started.Pumping the brakes seems to have little affect on the pedal.I can hold the pressure on the pedal for a good length of time with no pedal fade whatsoever...to me that seems to indicate that the M/C is OK ?I didn't accomplish anything in regard to correcting the bad brakes but out of frustration.... I did manage to put a nice dent in the hood

Did I mention I HATE BRAKES!!Guess after serving my 24 hour cooling off period....I'll have another go at it.Maybe I should farm this one out before I have to rebuild the entire car

Mikej · October 17, 2005, 09:30:24 AM

Daryle, One of the magizines last month had a article on this same thing. I'll see if I can find it. If you useing the a 1" piston master cylinder, it maybe to big. They also talked about the booster.

flt-blk · October 17, 2005, 09:50:39 AM

On my stude I went from a 1 1/8" bore to a 1" bore master cylinder and
it made a big difference.

I think the 1" is 78 Corvette, after they got rid of the big piston Delco
calipers. It took 3 tries to get the proper M/C at my Parts Plus store.

40 · October 18, 2005, 08:50:52 AM

Worked very late last evening to finish up a home we're building up on the river and didn't get back after the brakes.Since I don't feel real confident with the componants used in the set-up,I am leaning tward scrapping the entire system.I will replace it with the dual diaphram booster and go to a 1" or possibly a 7/8" bore cylinder....this will require revamping the mounting bracket and re-routing the brake lines but should give me the brakes I'm looking for.....I HOPE

Mike,If you can recall the mag and month that brake article is in I'd appreciate it.....I probably have the mag as I subscribe to nearly all of them....that said,they are stacked on the floor in the corner of my office,I haven't even looked at one since probably last February!

Mikej · October 18, 2005, 01:04:21 PM

Daryle, I found the article but I forgot which one it was again. I did leave it laying out. As long as the dog doesn't eat it today, I should be able to tell you tomrrow. It was not the same problem you are having but did suggest using a 1" MC and at least a duel 9" booster.
If you pump the pedal do you get a higher pedal?

HotRodLadyCrusr · October 18, 2005, 02:33:47 PM

Good problem solving brake post and perhaps should be included in the tech forum for furture reference once all gremilns are worked out successfully.

enjenjo · October 18, 2005, 02:39:23 PM

I just recently had a car with a similar problem, with every thing right, it just didn't have enough brakes. In this case, it had a 7" single booster, and a 1 1/8" bore master cylinder. After changing the booster, and master cylinder, with no improvement, I changed to a 7" dual diaphram booster, and a 15/16" bore master cylinder. that solved the problem.

by the way, if you have Wilwood residual valves, I have yet to see a 2 psi valve that would work at all. I will no longer use them.

Mikej · October 18, 2005, 07:34:25 PM

Daryle. The dog ate one of my parts books but spared the Nov 2005 Car Craft. On page #62 is the article that I think might help. The dog is a good hunter......really. Check your vacuum, at idle, in drive, with your foot on the brake. The article talks about a different problem than your but may shed some light on it for you. Do the the rear caliper have 2 bleed screws each? I have garage space if you get tired of it

40 · October 19, 2005, 01:18:46 AM

Thanks Mike...I've got that mag around here somewhere....I see Streetrodder did a multi-part article on brakes that seems to be well written and informative also....Now,all I need is the time to read them!!Also...Before I forget...I'm planning a surprise birthday party for the wifes 50th....I have rented the Knights of Columbus Hall near our house.It will be on Saturday,October 28th.I'll shoot you an e-mail with more info....it just popped into my head so I thought I'd mention it in case my "OldTimers" kicks in

Mikej · October 19, 2005, 09:26:00 AM

Daryle, Thats that opening day in Iowa and I've put up with this dog all year. I would hate to disappoint the dog but will see. Anita said her birthday was coming up. We were going to send her a half a dozen cards, just to remind her.

Kctom · October 19, 2005, 09:54:33 AM

Quote from: "40"Did I mention before that I HATE brakes??I'm trying to finish up my 36 and drive it a bit before the snow flies and am having trouble getting a good pedal.It is an under floor,4 wheel disc set-up with a 7" single diaghram power booster.I have 16 to 18 inches of vaccum,the correct residual valves,have bench bled the master cylinder and bled the system 3 times...once manually and twice with a power bleeder...once from the m/c outward and once from the bleeders in.Have I ever told you guys how much I hate brakes :x With the motor off,I have a great feeling/firm pedal but the moment I start the car....The pedal softens and goes ALMOST to the floor.I have adjusted the push rod to the point that I have just a slight touch of drag on the brakes.When I engage the pedal,all of the calipers seem to be moving the pads etc correctly...I can stop the wheels but it just barely holds the car still at idle and the pedal is within a 1/4" of the floor.Have I said before how much I HATE brakes! I am not totally sure of the bore size...I think it's an inch and an 1/8.I am also not real sure of the manafacturer as this setup came with a project I purchased...it was new in the box but had no info with it.It is the typical Corvette style setup sold by ECI,Master Brake etc.I am not a big fan of single diaphram boosters...I always order a dual but I have seen this identical set up used many times with good results and am thinking I may have a defective booster or M/C? Any thoughts or help will be greatly appreciated....Man,I HATE brakes

What kind of calipers do you have on the back?