Chevy Brake Problem

[buttanic]

It does this because the new pads are thicker and put the piston back into a spot that isn't smooth on the cylinder wall. Not a pad problem.

As far as what returns the piston? Duh, everyone knows that's the piston return Springs!

Please show me a picture of these piston return springs on a brake caliper. Having rebuilt many a caliper I am yet to find this return spring.
Drum brakes have return springs, they are necessary because of the self energizing design of drum brakes. Without the springs the brake shoes would continue to force themselves against the drum even if you release the petal.

 

[paddle007]

You asked for it. Kart rear brakes. The exception to the rule. What a mess!

 

[buttanic]

You asked for it. Corvette rear brakes. The exception to the rule. What a mess!

It appears they are using a design similar to the old drum brake wheel cylinder with a cup behind the piston rather than an O ring seal around the piston. It also has a quick change pad system where the retainer pin 17 can be pulled and the pads replaced without removing the caliper. I still don't get the 4 springs shone in 21. Is this a stock set up or competition. Doesn't look like a stock caliper.
Definitely not a set up you find everyday

 

[paddle007]

My flub. I went from Vette to Kart. My bad. At a glance I thought it was Stingray rear calipers because of the springs between the pistons. I've got to slow down when glance reading stuff.
That gen of Vettes have the floppiest pistons I've every seen. The lightest warping of the rotors bouncing against the piston will allow air into the caliper. Definitely little drag on those resting pads.

 

[SVTFreak]

Please show me a picture of these piston return springs on a brake caliper. Having rebuilt many a caliper I am yet to find this return spring.

Cmon man. All real mechanics know about piston return springs. All motors have one per piston else how do they return to the bottom of the bore?



It was a joke, dude. Those are usually commented about right after muffler bearings and blinker fluid.

 

[general mills]

It does this because the new pads are thicker and put the piston back into a spot that isn't smooth on the cylinder wall. Not a pad problem.

In my case, no 1 pad was out of spec. I swapped sides, and had the issue on the other side. I measured them with dial calipers and while I can't recall the exact thickness difference, it was significant, and the person on the help line phone agreed. The piston was fully seated on each caliper as I tried them. Easy enough to return them, I thought I would mention this as I hadn't seen this possibility mentioned yet, and I would want a new pad if I overheated them anyway.

 

[LNSvince]

A quick summary to describe whats going on :

The piston is drawn back in *an automotive* system because:
There "IS" a return spring in the Master Cylinder
This is a sealed system following the properties of Fluid Mechanics.
Positive Pressure is being applied from the pedal through the master.
When the Pedal is released and the applied pressure is lost,
that positive pressure is now at the piston. [pressure stays constant in a sealed system]
The pressure needs somewhere to go, therefore it will push back and be drawn up into the Master [ at the same time ] because the seals in the master cylinder are being driven in reverse.
Both the line pressure and the return spring in the master work together to draw the fluid up & release the piston.

Smaller systems can work without a return spring in a master. Because of paddle007 seal tech.
Every system Depends on how much fluid , weight of parts and mechanical movement that was necessary.

Remember this is a "summary". I've changed my wording a gazillion times. Each sentence must be taken as a general blanket statement. I realize we can dive off into this very quickly, debating every word, picking apart every line.

 

[LNSvince]

OH, if you smoked those pads once or twice. You should change them.

 

[paddle007]

Thanks Vince for covering the fluid dynamics. I did find a better explanation of the square cut seal than I could deliver. It even has pics. Check out this PDF. http://www.ucx.com/documents/squareseal.pdf

 

[LNSvince]

People wanted to see a return spring.
This is ONLY part of the "SYSTEM". Many more things going on..............
Pistons will travel PAST the point of square seal stretch in a panic situation. Square seal effect is there to stop pad dragging,aids in pad life and a MUST have component in a braking *SYSTEM*.

[video]http://upload.wikimedia.org/wikipedia/commons/4/47/Hydraulic_disc_brake_diagram.gif[/video]

 

[LNSvince]

Thanks for that Link.

I am going to print it out and poster it for customers to see.

I have the "The calipers are not leaking? I have NEVER changed calipers" conversation every day !


This is going to minimize "THAT" conversation.

 

[LNSvince]

Thanks for that Link.

I am going to print it out and poster it for customers to see.

I get the "The calipers are not leaking? I have NEVER changed calipers" every day !


This is going to minimize "THAT" conversation.

 

[buttanic]

People wanted to see a return spring.
This is ONLY part of the "SYSTEM". Many more things going on..............
Pistons will travel PAST the point of square seal stretch in a panic situation. Square seal effect is there to stop pad dragging,aids in pad life and a MUST have component in a braking *SYSTEM*.

[video]http://upload.wikimedia.org/wikipedia/commons/4/47/Hydraulic_disc_brake_diagram.gif[/video]

The animation is a little misleading as it shows the pads pulling away from the disc, this does not happen, at least to no measurable amount. Spin a wheel and you can hear and feel the slight drag of the pad against the disc.

 

[barbarossa]

Barbarossa, make sure you don't have a collapsed hose on that side, as I did years ago, with a '93 Chevy P/U I had. It caused so much heat that it permanently burnished the alloy wheel that was on that side. Apologies if anyone already suggested this.

I'm thinking that since it worked fine before the pad change, and then again after reinstalling the old pads, I can rule out a break line issue. Am I right?


Thanks to everybody for their input. I learned something.

Right now I'm still driving on the old pads, will do a caliper change when I have time.

 

[paddle007]

The animation is a little misleading as it shows the pads pulling away from the disc, this does not happen, at least to no measurable amount. Spin a wheel and you can hear and feel the slight drag of the pad against the disc.

But it is a measurable amount. The rotor does spin now, the piston/pads had to move.
Another factor is the amount of drag is also dependent on what residual pressure is still applied to the caliper. This varies but is typically just a few pounds. The master cylinder controls this unless it is a custom brake job. Those little blue, purple or red unions on the rear brake lines are residual valves. The are offered in various pounds of pressure. Put 15psi units on and the car becomes difficult to push around the shop. The 2 or 4's are hardly noticeable.

 

[buttanic]

Residual pressure valves are almost exclusive to competition cars, never seen any on stock cars. Racers want the shortest pedal travel and residual pressure valves provide it.

 

[paddle007]

Fords used them at the master cylinder port. Always wondered what they were there for.
If you go back to rear wheel cylinders it's even worse. It take 100 to 150 psi to move the brake shoes. The pressure is always there to keep the pistons against the shoes.
Link I copied from http://www.pirate4x4.com/tech/billavista/Brakes/
Built-in Valving

This one can be a deal-killer. If the MC in question was designed for disc/drums and has a built-in residual pressure valve, it will not be suitable for disc/disc brakes. See section below on valving for description of residual pressure valves. For now, the point is, know the MC in question and whether or not it has built-in valving (many do not). If it does, you would have to either modify it by removing the residual pressure valve to make is suitable for use with disc brakes, or choose a different master cylinder.

Valving

Residual Pressure Valve

A residual pressure valve is a simple, one-way, spring-loaded valve installed either in the master cylinder, or inline between the MC and the callipers/wheel cylinders. They operate by keeping a pre-determined amount of pressure in the brake lines, even with the brakes released. The internal spring determines the amount of residual pressure kept in the brake lines – normally 2 PSI or 10 PSI. There are two distinct uses of residual pressure valves:

10 PSI: Drum brakes only. Because drum brakes don’t use callipers and are therefore not self-adjusting there are springs installed to retract the brake shoes away from the drum. A 10 PSI residual pressure valve is used in drum brakes to keep a little pressure in the lines to balance the return-spring force so that the shoes are maintained in close proximity to the drums. Without the residual pressure valve, the return springs would retract the shoes so far from the drums that excessive pedal travel would be required before the brakes are applied.

2 PSI: Disc brakes only. In many race cars and hot rods, the master cylinder is installed at a level below that of the callipers. As such, gravity will draw the fluid from the callipers, causing it to drain back into the MC. The result is a “spongy” feeling pedal and excessive pedal travel. A 2 PSI residual pressure valve is installed in the brake line between MC and calliper to maintain slight pressure in the line and prevent fluid drain back. Note: This valve should only be needed if the MC is lower than the callipers.

I have often read where people have advised the use of a residual pressure valve to cure some other problem - spongy pedal or excessive pedal travel (usually caused by insufficient volume from a too-small MC). In cases like this, it may feel as if the RPV has cured the problem, but it is only a band-aid, masking the real problem. Use an RPV only for drum brakes or MC lower than callipers. Do not use an RPV as a band-aid. Find and fix the real problem!

When converting from drums to discs, you will need to remove any RPV in the rear circuit.

 

[buttanic]

Good web site.

 

[general mills]

Vince, good animation. I always pictured the return spring in the master cylinder there to return the pedal when you let off and the system is now at 0 psi. I guess I figured the piston was somehow ported to prevent pulling the pads off the disk on the return stroke? On the pad side, I always pictured it as pressure on, pressure off, and that is it. Once again you grace me with the light of knowledge

 

[LNSvince]

Sorry to Hijack , so please PM me and do not reply to this thread.
If there are any TECHNICIANS roaming here.

I am looking to hire someone. $35 hour commission.
Sorry guys, but at this time, I can not take on a trainee.
We are just too busy.
I need a full blown tech that is capable [ and not intimidated ] of working on ANYTHING.
You do not have to be a master tech.
I can teach someone with experience.
But I do need someone with a few years under your belt.

Confidential interviews.

Vince 225-405-5199