[DeTomaso] Wilwood Brakes WTF????

Tomas Gunnarsson guson at home.se
Thu Apr 18 16:02:38 EDT 2013



So it's a race between the spring in the MC pushing the MC piston back
and the caliper pistons pushing fluid back through the lines?  
 
>No, the calipers don't actively push fluid back at all. Theoretically
there is no movement of the piston, just a pressure rise. When you let
off the brake the seals stop sealing against the master cylinder and
piston and allow the piston to return and should there be a void in
front of the piston it will be filled from the reservoir. This is how
the system self adjusts to pad wear.
 
I need to look at my brakes, I had thought that if I jack up a corner
and press and release the brake pedal, the caliper pistons would
retract.  Are you saying that in the absence for the rotor
pressing/rubbing against the pads, the caliper pistons will not retract
into the caliper?  I guess I assumed they would, without ever thinking
about it too much.

>Correct. You may see a small movement as the caliper "yawns" as it
flexes du to the pressure.
 
If fluid can never flow back past the MC piston into the reservoir, why
do you need a residual pressure valve to keep from losing fluid from the
MC and lines, which results in the pedal going to the floor?  Does some
fluid sneak past the MC piston over time and the residual pressure valve
prevents that?

>It can, see above.
 
Tomas


________________________________
From: Tomas Gunnarsson <guson at home.se>
To: kenn_green at yahoo.com 
Cc: detomaso at poca.com 
Sent: Thursday, April 18, 2013 10:12 AM
Subject: Re: [DeTomaso] Wilwood Brakes WTF????


The master cylinder piston and seals are designed so that the sealing
function is lost when the piston travels back from the "brakes on"
position. Also, the seals allow fluid to run past them as long as the
brakes are not applied. 

Tomas

<-----Ursprungligt Meddelande----->
From: Ken Green [kenn_green at yahoo.com]
Sent: 18/4/2013 6:11:31 PM
To: Tomas Gunnarsson; detomaso at poca.com
Subject: Re: [DeTomaso] Wilwood Brakes WTF????

Isn't there a spring in the master cylinder that retracts the MC piston
and therefore also the caliper pistons?  I had assumed that the residual
pressure valve worked against the spring in the MC?  If you look at the
reservoir ports in the MC there should be a relationship between port
position, piston travel, and transferring fluid from the reservoir to
the MC bore and brake lines.  Somehow, adding the residual pressure
valve allows more fluid to enter the MC bore, otherwise, I don't see how
you end up with shorter pedal stroke?

Ken


________________________________
From: Tomas Gunnarsson <guson at home.se>
To: detomaso at poca.com 
Sent: Wednesday, April 17, 2013 11:49 PM
Subject: Re: [DeTomaso] Wilwood Brakes WTF????


Nothing. There's no mechanism actively retracting the pistons and that's
why the valves are not needed. The only retraction you should see is
rotor kickback and the flex of the piston seals. If you have excess
kickback that's a problem in itself, nothing you "fix" by adding pad
preload with valves.

Tomas

<-----Ursprungligt Meddelande----->
From: richard bosch [theemonkey at yahoo.com]
Sent: 18/4/2013 2:52:25 AM
To: Tomas Gunnarsson
Cc: detomaso at poca.com
Subject: Re: [DeTomaso] Wilwood Brakes WTF????

So what keeps the pistons from retracting too far?
Just curious how it is designed without the valves.
--- On Wed, 4/17/13, Tomas Gunnarsson <guson at home.se> wrote:

From: Tomas Gunnarsson <guson at home.se>
Subject: Re: [DeTomaso] Wilwood Brakes WTF????
To: theemonkey at yahoo.com
Cc: detomaso at poca.com
Date: Wednesday, April 17, 2013, 2:08 PM



No, I have disassembled both master cylinders and calipers down to bare
castings and there are no valves.

Tomas



<-----Ursprungligt Meddelande----->

From: richard bosch [theemonkey at yahoo.com]

Sent: 16/4/2013 10:52:14 PM

To: Tomas Gunnarsson

Cc: detomaso at poca.com

Subject: Re: [DeTomaso] Wilwood Brakes WTF????









It could be that only the American cars have the valves in the master
cylinders.  



Maybe Europeans have them in the calipers or distribution block?



They have to be somewhere in the brake system or else you have a delay
in the brakes being applied as the pistons would have a much longer area
to traverse before they contact the rotor.










--- On Tue, 4/16/13, Tomas Gunnarsson <guson at home.se> wrote:




From: Tomas Gunnarsson <guson at home.se>

Subject: Re: [DeTomaso] Wilwood Brakes WTF????

To: theemonkey at yahoo.com

Cc: detomaso at poca.com

Date: Tuesday, April 16, 2013, 6:06 AM






Must be an American thing. Euro master cylinders I've come across have
no valves built into them.

Tomas



<-----Ursprungligt Meddelande----->

From: richard bosch [theemonkey at yahoo.com]

Sent: 16/4/2013 1:10:46 AM

To: detomaso at poca.com

Subject: Re: [DeTomaso] Wilwood Brakes WTF????




Disc brakes require 2 psi residual pressure valves.Drum brakes have 10
psi. valves.

Most master cylinders have them under the flare seat  where the brake
lines are screwed in thats why you do not normally see them or think
about them.

Without the residual pressure the pistons retract much further in the
caliper and there is a delay while the piston has to travel further
before it contacts the rotor.Not good as seconds count in braking
distance!

Mike's valves are "add ons" because his master cylinder does not have
any valves built in.His valve pressure is correct as the pads did not
drag on the rotors before the pad wear/cocked pad happened.

Just what i have learned from playing with lots of cars  :  )

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