Brake issues????

[wiedemab]

A friend of ours just put together a '67 Fairlane. It has granada front discs and Explorer rear discs on a 9". He has the following Wilwood MC
http://www.wilwood.com/Products/006-...IRMC/index.asp

The problem that he is having is that the first pump of the brake pedal goes to the floor - the second pump is firm and the car stops great. We have basically started over with benchbleeding the MC all the way through bleeding the entire system. He is still having the same issue.

Any suggestions of what to try next?



Thanks,

Brandon

[6'9"Witha69]

What about repeated back to back stops? Does the first pump soft second pump good still apply? If not, it is just when the brakes are unused for a short period, then residual valves may be needed.

[wiedemab]

I just called him to ask exactly how it is acting. He drove the car a little bit last night. He says that the pedal will stay firm on back to back stops, but after a while it will have to be pumped up again.

I've read about the use of residual valves. Everything I read is that they are used when the MC is lower than the calipers. This master cylinder is mounted on the firewall significantly higher than the front and rear calipers.

It acts to me like it may be drawing air into the system somewhere. The symptoms sound like air in the system, but they've bled it. I'm goinig to try to go to his house tonight and make sure that is being bled correctly.

Is there anything related to the master cylinder that could be causing the issue - it is brand new, but I've had issues with new parts before.

[6'9"Witha69]

My MC is above the Calipers but I still had to use a resid valve on the front lines. Back to back was great. Go a few blocks between and it started to go soft. If it sat for a week, there was no pedal at all and it took 3-4 pumps to get it back.

I suggest the RPV with that MC (same as mine).

[wiedemab]

I'm willing to give anything a try. I have residual valves for my '52 Chevy Pickup. I think the front is a 2lb (disc) and the rear is a 10lb (drum).

Would you suggest trying the 2lb RV in the front or putting a RV in the front and rear?

2lb on both??

Thanks for you help!

[6'9"Witha69]

2lb are for discs. 10 is too much far any disc line. I found it was my fronts and that did the trick. Some people need both. Try the fronts first.

[wiedemab]

I'll take him my 2lb residual valve tonight and give it a try in the front. Thanks a bunch for you help. I'll update the thread and let you know how it worked out.

Later,

Brandon

[70 LS1]

So what causes this? Is it a knock back issue or is it just fluid draining out of the calipers?

[6'9"Witha69]

It is fluid draining back. Knock back is a mechanical issue where the rotor actually kicks or knocks into the pad, causing the piston to retract more and forcing fluid back upstream.

[MonzaRacer]

Well in actuallity the system needs to retain some pressure to keep the pistons from going back too far. See the pistons in disc calipers ar pulled back by the seals, basicly they are a square cut seal set at a slight angle so the as the pistons come back as theseal tries to straghten up to its regular shape, but while they use aseal that can bring the psitons back they will keep moving, the system/fluyid can cool and cause too much retraction. So you install a small check valve to leave a small amount of pressure in the line so the pistons do move back too far on thier own.
Generally Knock back in my world(as a tech) is one of the ways a warped or excessive DIR(dial indiacted runout)plagued rotor or wheel bearing very loose too can cause that.
2lb residual valves and an adjustable prop valve and your good to go.
The trick to adjusting one in my experience is to keep adding rear pressure till your car stops neutral and you even feel the rear pull down on a stop. Most cars are biased too far to the front.
Ha d a fella ask my boss if we would work on his aftermarket system ,he thought the master cylinder was bad. so I got it all checked out, rebled the system and then took it out and started readjusting it. The car went from a bear to wrestle down to stopping on a dime ,litterally.
My boss came in afterdriving it rubbing his forehead. He smacked it on steering wheel when he panic stopped!
Owner came in and was shouting at the top of his lungs ad I was worried something had broke.
He comes in and is babbling so fast all I got was Brakes and OH MY GOD.
So he calms down, and tels me that the cop who stopped him for an unrelated reason almost rearended him as he was tailgating too close.
Cop was bitching that the owner was trying to get him into a wreck. And he explained he had just had me work on the system and it really stopped good now and he wasnt used to it yet.
He was thrilled and couldnt believe he had the car for 3 yrs and it had never stopped so good.
Good luck