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> Front brake calipers dragging, Fresh rebuilt, just bled
VaccaRabite
post Jul 18 2008, 10:48 PM
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So, using the long tubes method, I got my brakes bled tonight. The pedal is *really* solid.

But, when I went to each caliper, the Drivers front caliper was tight against the rotor. Passenger front was dragging, but not as much. Both rears were dragging a little. On each rotor I was able to turn it, and it started to come free. However, especially the drivers front, they are not free wheeling.

The rears are easy, I just re-vent them.
The fronts, though. Whats going on there?

I was really stomping on the brakes getting the prop valve bled. I can't imagine that I damaged something. According to Haynes there is no front adjustment. What is to keep them from dragging?

The pedal cluster has also been rebuilt and bushed with bronze bushings. Everything with the brakes has been renewed (rotors, pads, calipers [ E.Shea in the back, I rebuilt the front] hard lines, soft lines) except the prop valve (which does not need it).

Zach
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ejm
post Jul 19 2008, 01:49 PM
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Is the pedal push rod free play correct? Had a car at the WCR with the same problem.
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Eric_Shea
post Jul 19 2008, 01:51 PM
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QUOTE
The fronts, though. Whats going on there?


Can you describe your rebuild process? Did you clean out the knock back mechanism's in the pistons?

The fronts will rely on a bit of runout to knock the pads off the rotor. Are you driving this or is this just garage testing at this point?

Make sure you adjust the rears on the high side of the runout and adjust to .004"
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VaccaRabite
post Jul 19 2008, 01:59 PM
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QUOTE(Eric_Shea @ Jul 19 2008, 03:51 PM) *


Can you describe your rebuild process? Did you clean out the knock back mechanism's in the pistons?

I got a rebuild kit from Pelican and followed the instructions it had. I did not pull the pistons out, as the instructions said it was not required.

Calipers were split. All the rubber bits removed. All passages were throughly cleaned with brake cleaner and compressed air. All the kit parts put in as per the instructions.

QUOTE(Eric_Shea @ Jul 19 2008, 03:51 PM) *

The fronts will rely on a bit of runout to knock the pads off the rotor. Are you driving this or is this just garage testing at this point?
Make sure you adjust the rears on the high side of the runout and adjust to .004"

Just garage testing. Engine is still sitting on the test rig waiting for me to finish syncing the carbs and getting the electrics fully installed in the engine bay.

What is "run out?" Does that mean that the fronts are designed to get knocked free by the wheel spinning?

Zach
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Eric_Shea
post Jul 19 2008, 02:03 PM
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I did not pull the pistons out, as the instructions said it was not required.


HUH? (IMG:style_emoticons/default/confused24.gif) You mean the knock back mechanisms in the pistons??
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VaccaRabite
post Jul 19 2008, 02:11 PM
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QUOTE(Eric_Shea @ Jul 19 2008, 04:03 PM) *

QUOTE
I did not pull the pistons out, as the instructions said it was not required.


HUH? (IMG:style_emoticons/default/confused24.gif) You mean the knock back mechanisms in the pistons??

I'm sorry, I'm noob at this. I am looking at the exploded diagram of the front caliper in th Haynes manual, and I don't see the knockback mechanism that you are talking about.

The rebuild kit was just new seals and such.

Are you talking about the spreader spring that sits between the pads? If so, that is in place.

Zach
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Eric_Shea
post Jul 19 2008, 02:20 PM
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Let's start with the pistons. If you did not pull the pistons as you mentioned then this is your problem. The pistons and the bore they sit in are the essence of the rebuild. So; did the pistons come out? Did you hone the bore?

The knock-back mechanism is the mechanism inside the piston that rides up and down on the metal shaft that resides in the middle of the piston bore. If that was not cleaned thoroughly during the rebuild process this may be your cause.

If you didn't pull the pistons you would have two major seals left over.

Not talking about springs or piston positioners... just pistons and their internals.
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VaccaRabite
post Jul 19 2008, 03:29 PM
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I did the rebuild over the winter, and I know that I did not have any parts left over. And thinking about it now, I do remember removing the pistons and looking in the bore.

However, I know that I did not hone them as I do not have a tool to do that.

My personality is such that if the instruction sheet said to hone the bore out, I would have gone and gotten the tool to do it. I suppose this means that I need to rebuild my front calipers again?

Zach
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Eric_Shea
post Jul 19 2008, 03:50 PM
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Hard to say. It sounds like they're hanging up a bit.

www.mcmaster.com has the hone for $13.00.

I'd be inclined to blow the pistons back out, polish them on a bench wheel and hone the bores. Clean out the knock-back mechanisms at that time as well. You can probably get by using the same seals. While they're apart, take them to a local plater and have a new coat of zinc put on them.

Runout is normal "wobble" in the rotor BTW. It should be within spec. but too much can be bad.
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Rav914
post Jul 19 2008, 08:16 PM
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Mind if I jump in? Eric, how do you clean the knock back mechanism? Anything special?
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Eric_Shea
post Jul 20 2008, 02:16 PM
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For 914 pistons I recommend brake cleaner and compressed air. For 911 pistons I recommend taking them out (very easy) and then the same method.

914 pistons are very difficult, if not impossible to get the mechanism out. The clips they use are fairly permenant and there aren't any new ones readily available.

A good solvent soaking and compressed air is usually all that is needed.
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Rav914
post Jul 20 2008, 04:00 PM
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Thanks Eric. I ask because I just ordered a rebuild set (front/aft) from you and will be doing this job shortly. You did the brakes on my '74, which are flawless, but I'm doing this car on the margin and need to DIY as much as possible.

Sorry for the highjack.
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melnyk
post Jul 26 2008, 06:38 AM
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QUOTE(Eric_Shea @ Jul 19 2008, 05:50 PM) *

Hard to say. It sounds like they're hanging up a bit.

www.mcmaster.com has the hone for $13.00.

I'd be inclined to blow the pistons back out, polish them on a bench wheel and hone the bores. Clean out the knock-back mechanisms at that time as well. You can probably get by using the same seals. While they're apart, take them to a local plater and have a new coat of zinc put on them.

Runout is normal "wobble" in the rotor BTW. It should be within spec. but too much can be bad.

another highjack.
what size hone would i need and what grit? the finer the better? whats the best way to polish the pistons and if theyre frozen its it ever impossible to get the pistons out?
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davep
post Jul 26 2008, 07:08 AM
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The size of hone depends on the bores you are working on. The 914 can have bores from 33mm to 48mm or about 1.25" to 2". The triple stone hone is what I use. Be sure to get a hone that you can get replacement stones for. I like the shorter stones myself. Finer grit is better. You are not looking to remove material from the bore, just to clean it.

Removing stuck pistons requires some ingenuity and a hydraulic system. The easiest hydraulic system is your car. Some grease guns have the correct fitting (10mm by 1.0 pitch) for the fluid inlet. Ingenuity is required to hold the moving piston in place (sealed) while forcing the stuck piston to move. If they are bad, then send them to a pro with the tools and experience to do the job without doing damage (to person or parts).

Getting the pistons cleaned up can be tricky. A very mild acid like citric acid can be used, but it can be hard to obtain. A green scrubby such as used for washing dishes can be used to polish the piston sides. Brass wool can be used. Under no circumstances use steel wool, sandpaper or emery cloth. When Eric says to polish on a bench wheel, I believe he is talking about a cloth wheel, not a grinder or wire wheel. Basically you cannot use anything as hard as the steel for cleaning. If the piston is pitted, then replace it.
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melnyk
post Jul 26 2008, 07:22 AM
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well im redoing all of my calipers do you know what the bore sizes for the front calipers and the rear calipers are?
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davep
post Jul 26 2008, 09:46 AM
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42 & 33mm I'd guess; you don't say what you car is...
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melnyk
post Jul 26 2008, 11:31 AM
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QUOTE(davep @ Jul 26 2008, 11:46 AM) *

42 & 33mm I'd guess; you don't say what you car is...
oh sorry its a 73 914
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davep
post Jul 26 2008, 12:00 PM
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Please add the VIN and other data to your profile. Every car counts.
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Eric_Shea
post Jul 26 2008, 04:34 PM
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Get the hone here:

www.mcmaster.com

6713A12
Clean-N-Polish Hone for Small Cylinder for 27/32"-2" Cylinder, 3 Stones, Medium 240 Grit
In stock at $14.84 Each

- or at your FLAPS for about $16.00. This will work with both your rears (33mm) and your fronts (42mm).


Get this grease gun:

http://www.harborfreight.com/cpi/ctaf/disp...?Itemnumber=219

- and remove the tip. The hose then screws into your caliper and you can literally pump the pistons out with the grease. I like Dave's "keep them on the car" tip as well. It cleans up much easier. Regardless you will have to formulate a rubber plug for the open piston bore once it comes out.

Cloth Wheel - yes, we use a cloth wheel on a bench top buffer and a mild polish to get the pistons to a mirror finish.

When you have the calipers out and apart you may wnt to consider finding a local plater to put a nice coat of zinc back on them:

http://maps.google.com/maps?rls=com.micros...esults&cd=1
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914werke
post Dec 18 2011, 03:41 PM
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QUOTE(ejm @ Jul 19 2008, 11:49 AM) *

Is the pedal push rod free play correct? Had a car at the WCR with the same problem.


good question WHAT IS the free play supposed to be on the plunger arm ?
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