not pad knockback, or is it?

[ehummelman]

I have read and read about brake pedals that are soft at first but firm up on the 2nd pump, and it seems they all talk about pad knockback. From what I gathered, this typically affects cars on track under high corner loads, causign the rotor to flex or move pushing the pads back into the caliper. I'm not running on track or cornering on the street at any kind of speed/side load, but yet I need to pump before approaching a stop sign or other obstacle. It did not happend when I got the car earlier this year, but has been getting fairly noticeable as of late. I would think that if it were just in need of a fluid flush or bleeding that the pedal would be soft all the time and not firm up on the second pump. Am I really experiencing knock back? I've never had fixed calipers so this is a new one for me. If it matters, they are Wilwood 6 pistons in front and 4 pistons in back.

[Vegas69]

If your pedal stays high and firm when you drive in a straightline and then drops after going around a corner, you have brake pad knockback. If the pedal is low right from the start when you drive, you have a fluid or bleeding issue. My car would experience knockback under normal driving conditions when I first built it. The only way to fix it totally is to change to a floating caliper on your rear axle or to a floating axle. You can improve it by going to tapered axles, larger bore master cylinder, shimming the bearings, etc. but it's just a band-aid. These cars should've NEVER had fixed rear calipers. They simpy aren't engineered for it. Wilwood has a kit out now to change over to a floating caliper that keeps the internal parking brake. Very happy with mine.

[Apogee]

Pad knockback tends to be fairly consitent and caused solely by certain manuevers. Since your brake performance has been steadily declining for a while, it sounds like you have air in one of your brake circuits. You could do a quick bleed by axle to determine whether it's the front, rear or both causing the problem. Whatever the problem is, if you don't address it now then you'll just be chasing the same thing later, so once you know which end(s) it is, check for leaks or any other potential issues.

Tobin
KORE3

[gsxrken]

. Wilwood has a kit out now to change over to a floating caliper that keeps the internal parking brake. Very happy with mine. You can call Dustin Burr over at Wilwood. He's a great guy.

Is this some sort of bracket or is it a different caliper? My rear calipers mount onto the ears of the wilwood parking brake assembly very much like in the below pic I found on another site:

[Vegas69]

It's a different bracket and caliper.

[MonzaRacer]

As for low on first pump install 2 psi pressure checks in all the lines or in the lines out of the master cylinder. Well thats if bleeding doesnt work for you.

[Vegas69]

That only works on a master mounted below normal height. 2psi residuals don't do a damn thing for knockback.

[MonzaRacer]

Who says its knockback? After having fixed several modified systems that had 4 whl disc replacing stock parts by adding these simple parts I can tell you it also works in certain systems where the calipers are actually pulling the pads back too far and it takes more pedal stroke to make them touch. I actually had a guy using Porsche and Volvo calipers in a circle track car, he loved having 4 wheel brakes, and having adjust-ability in his system and he also had handbrake control over specific wheels, more for parking but also for fun.
He had an issue that the year of Porsche he used had built in more pad retraction and it allowed it to have nearly no pedal regardless of pedal ratio or master cylinder sized used 2 psi on one end and 10 psi on other and tada no more low pedal. Easy way t ofind which end needs it if it happens at rest, if you can swap in some rubber stock type lines good enough for test, pump up system and clamp off line, let set if wheel stays locked and pedal stays high you have found axle with issue. rear with single center line are easiest to test first.
not saying ti will fix it but did in one car after about a month of investigating WHY it was acting odd, then trying them and THEN finding out WHY after we fixed it. Turns out Porsche had an issue with pad drag so had calipers redesigned and not know this cause his issue it was two year design caliper and all before had not had issue, article he read gave years, he found these in German car salvage yard on shelf smae mounting points, same piston size so he got them cheap.

[ehummelman]

Thanks for the feedback guys. I tried to bleed all 4 wheels this weekend but it made no difference. Of course, that doesn't mean much since I may not have done a great job of bleeding. I used the Phoenix Injector reverse bleed procedure from the caliper end. It usually works great, but who knows. It's just strange that the second pump is always nice and hard. It makes no difference if I'm going straight or around turns, I have to pump twice now every time I need to slow down.

I noticed something else really weird, but not sure if it's normal or faulty. When I took some fluid out of the front master cylinder well prior to reverse bleeding, the level of the rear well went down with it. Likewise, both wells filled equally when reverse bleeding front/rear circuits. Aren't they supposed to be separate circuits, i.e. front well goes to only front circuit)? Otherwise why have a front and rear well? This is a wilwood mc btw, manual brakes.

[Payton King]

You may have a bad master. I had a Tilton go out on me after a very short time. One of the ways I checked it was the pump the pedal up and hold it with lots of leg presure. It would slowly go down over 30 seconds or so since the master was bleeding the pressure off.

[ehummelman]

I'm starting to think you may be right. The fact that the front and rear bowls do not seem to be separate makes me wonder if there's an internal leak. Aren't the front and rear circuits supposed to be completely separate? If I remove fluid from the back the front should not be affected at all, but like I said the levels moved up and down together. I've emailed Wilwood and am hoping they will confirm this is the problem. I should be able to just get a quick rebuild kit if that's the deal.

[Apogee]

You didn't specify which Wilwood master cylinder you're running, but if it's one of their die-cast aluminum models, there is a hole through the divider in the reservoirs that will level the fluid assuming the level is at least above that cross-over hole. While the reservoir is common, the two circuits should be isolated from one another once the piston seals clear the ports accessing the reservoirs. Any amount of fluid bypassing the piston seals internally would necessitate a new master cylinder and/or rebuild kit.

Tobin
KORE3

[ehummelman]

Thanks Tobin, I didn't know they had that feature. That must be what's happening, but I'll look to see if it has that hole. I have the standard 7/8 aluminum tandem that most guys are using with manual brakes. I have their 6 piston fronts and 4 piston rears to go with it. The previous owner bought it as a matched kit frm Frank at Prodigy a few years back. Well, I guess it's just my poor bleeding skills then. I'll keep trying to see if I can get it firmer. Perhaps I should flush all the fluid, maybe it's just absorbed some water over time.

[BADDRIDE II]

My $.02......either MC is bad or the system has not been bleed 100%.

The routing of the plumbing is critical. Believe it or not, you can create a nightmare for bleeding your system by improperly routing brake lines.
Never make any sharp up and then down turns. Fluid must move smoothly forcing the air in front of it to the calipers for removal. Fluid
will move up the line and then roll under the air bubble in a sharp loop, leaving the air trapped in the line (spongy pedal). If your braided/soft line at caliper has an up and down loop, pull it over horizontally during bleeding to help the air move to the caliper. Good luck!