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Old 07-13-2009, 03:36 PM
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Puzzling Brake issue

Although certainly not a brake expert, I've mechaniced all my life, and a hydraulic brake system is really not rocket science. Suddenly, I've got a brake problem that seems to be eating my lunch, and perhaps some of you with more brake experience have seen this happen before.

Originally I installed a front clip from a 78 Chevy Caprice in a hot rod. Utilized the original Chevy booster and master cylinder as well as the front discs/calipers. Worked just fine for 3-4 years.
Decided I needed to "dress things up" a bit under the hood, so purchased (from Ebay) a chromed chevy nova booster and master cylinder kit, new. Installed the kit, but found that there exists both deep and shallow rod bores for GM systems, and as luck has it, my old rod was long and the new one short. I modified my old one to the length that just barely applied pressure to the piston when retracted, and allows 1-1/2" or so of rod travel. Even though we bench bled the MS, we could not get it to pump fluid very effectively. Mounted it up, and attempted to bleed by both pumping through the bleeders as well as by using a vacuum pump at both the caliper and right at the MS. Seemed to eventually get all of the air out, but just didn't really seem right.
Brakes just were not as firm as before...pedal seems to go down too far, and eventually I gave up and replaced just the chrome MS with the orginal. Everything seemed to then work ok. So...not satisfied to leave the ugly old thing on, I tried once again with the chrome MS. This time, success, it worked as well as the original.
Except....only for a while. Seems that the right front caliper would periodically drag...squeak, or even at times get warm and smell. It just was not releasing completely. So...a brand new set of calipers/cylinders/pistons on both sides, hoping to cure the problem. Still the same...even the new one would drag at times. At times it got very severe...you could feel it dragging, and the rotor would smell hot. Strangely, the pedal would be very firm...could hardly move it 1/2" in travel. By either "jolting" the pedal or pressing rather firmly for several seconds, it would release, but then everything would swing the other way...barely any brake, and only way at the end of the pedal travel.
So...with new MS & booster, new calipers, good lines...seems that the proportioner valve must be at fault. Seemed to stand to reason the way it acted. SO...new adjustable proportioner was installed, new lines from MS to it, and all 4 brakes rebled until several ounces of fluid had been discharged from each. Unfortunately, the problem reoccured after only a couple hundred miles...same symptoms. I've tried shimming the MS mount out away from the booster a couple washer thicknesses to make sure the stroke is allowing the MS piston to retract completely, no change. I keep going back to the fact I never felt the new MS was pumping like it should when we tried to bench bleed it, and I can't help but suspect I've just got a defective master cylinder. Can't really see anything else it can be! When it works right, it works very good. Obviously my pedal travel or other factors are fine. Can't see a defective new caliper being likely, and the new proportioner is pretty straightforward...shouldn't be responsible for these symptoms the way I see it. Its just like there's a valve within the MS that is not seating to allow pressure to be built, and at times not releasing to let it return again. Is this a possibility? Thanks for any suggestions...

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Old 07-13-2009, 04:14 PM
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Is the right front the only one draging. If so I would check the hose. It may be collapsing on the inside. I have had a new hose do this to me. If that is the only wheel that is locking up I would think it has to be something in the right front. Is the caliper hardware good if not it may be letting the caliper stick at times. Are the slides clean. I Use a very small amount of seal glide on my slides. If it is a hub rotor are the bearing tight if not it will let the rotor float in an out on the caliper.
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Old 07-13-2009, 04:38 PM
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You also might want to check and see if the caliper on that side is mounting up correctly, or in some kind of a bind this will cause a drag and it to heat up also. do you have the proportioning valve adjusted correctly? Cole
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Old 07-13-2009, 10:44 PM
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Naw, the caliper seems to move as it should. The tell-tale sign is the fact that the pedal is nearly immobile when this happens...it will only easily travel a half inch or so, and all brakes are right there. Its as if there is pressure built up in the system that can't return to the reservoir when it happens. Was the same before and after the new caliper was installed, and nothing seems out of place with the mountings. Jack up the front end when its normal and the wheel spins freely, bearing seems properly loaded. 80% of the time, its working ok...its just when it decides to not release, we've got problems. Not sure why I'm not seeing the driver side grab as well...the old pads showed one (on the pass side) fairly well worn and the other 3 not hardly worn at all when I replaced them and the calipers. Again, once the system "lets go" and releases the pressure, the pedal goes almost all the way to the floor, and there is very little brakes. Pumping doesn't seem to help, just slowly cycling it and running it (carefully!) for a few miles seems to bring it back to normal to slightly weak. Could go 2-300 miles or several hours before it gets back to its dragging mode.
The proportioner is pretty straight forward...front in and two outs, rear in and one out. Pretty hard to screw that up I'd think. It could be possible that I've got the MC port for the front mixed up with the one for the rear...would that have any possibility of causing these symptoms? Seems I've heard different opinions on which is which...if I'm not mistaken, I've got the reservoir and port closest to the fire wall plumbed to the front brakes on GM...is that correct or backwards? The larger 1/4" line is from the forward-most port and feeds the rear brakes port on the prop valve. Is that right or wrong???
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Old 07-14-2009, 07:37 AM
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Quote:
Originally Posted by spika
Installed the kit, but found that there exists both deep and shallow rod bores for GM systems, and as luck has it, my old rod was long and the new one short. I modified my old one to the length that just barely applied pressure to the piston when retracted, and allows 1-1/2" or so of rod travel.
The "fix" you did is where your problem is. Modifying the rod to make it shorter is correct. BUT... there should be a slight amount of clearance between the end of the pushrod and the bottom of the master cylinder piston. This is so that the piston can fully return after being applied. If it does not fully return, there will be some residual pressure in the braking system that can and will cause one or more of the calipers or wheel cylinders to "drag". This will cause overheating of the brake(s) and or complete locking up. Another fix would have been to get the master cylinder with the deep hole for the pushrod that matched the pushrod you had.

All the other changes were probably unnecessary.

NOTE:

The master cylinder with the deeper pushrod hole is usually for a manual brake system. The master with the shallow pushrod hole is for a system that has "power" brakes with a vacuum booster.

On a dual reservoir master cylinder; if one of the reservoirs is larger than the other, the larger one is usually plumbed for the front calipers. The smaller reservoir is usually plumbed for the rear drum brake wheel cylinders.

Last edited by Frisco; 07-14-2009 at 07:43 AM.
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Old 07-14-2009, 05:19 PM
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I believe Frisco is on the right track.

Here it is boys, if you change a part and the system doesn't work, why in the world would you start changing OTHER parts?????? I am stumped at the thinking when people do this.

I have seen it all my life, the car gets a new distributer cap and wires during a tune up and the car then back fires and spits and the guy thinks the timing chain needs replacing! NO KIDDING, I HAVE SEEN THIS EXACT EXAMPLE.

The ONE thing that is the constant throughout this whole story is the rod.

Brian
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Old 07-14-2009, 05:50 PM
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Quote:
Originally Posted by MARTINSR
I believe Frisco is on the right track.

Here it is boys, if you change a part and the system doesn't work, why in the world would you start changing OTHER parts?????? I am stumped at the thinking when people do this.

I have seen it all my life, the car gets a new distributer cap and wires during a tune up and the car then back fires and spits and the guy thinks the timing chain needs replacing! NO KIDDING, I HAVE SEEN THIS EXACT EXAMPLE.

The ONE thing that is the constant throughout this whole story is the rod.

Brian
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Old 07-14-2009, 10:53 PM
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Quote:
Originally Posted by MARTINSR
I believe Frisco is on the right track.

Here it is boys, if you change a part and the system doesn't work, why in the world would you start changing OTHER parts?????? I am stumped at the thinking when people do this.

I have seen it all my life, the car gets a new distributer cap and wires during a tune up and the car then back fires and spits and the guy thinks the timing chain needs replacing! NO KIDDING, I HAVE SEEN THIS EXACT EXAMPLE.

The ONE thing that is the constant throughout this whole story is the rod.

Brian
I don't mean to sound contrary, gentlemen, but your logic has a peculiar smell. Consider the fact that the system works as expected 80% of the time. The length of the rod is a constant...it either is long enough or it isn't...that will not change. As mentioned, I tried a test just to see how I could effect it...with no shims between the mc and the booster mount, there was approximately 1/8" interference...the piston was "preloaded" when the pedal was fully retracted. With one set of flat washers, there was zero lash or preload; with 2 on each bolt there was enough play that you could sense the pedal having to travel farther before the piston was engaged. No difference was noted in the core problem, however, at either setting.
As for the other parts...I may have not mentioned that this is a 30+ year old system that sat unused for a number of years. The calipers & pistons, master cylinder, booster, and even the proportioner were certainly showing their age. If I'm going to trust the system to stop me from highway speeds, I believe it makes sense to go with new.
I've dealt with hydraulic systems of various types over the years, and this sure seems to be a case of a check valve sometimes seating sometimes not. That is something that can be intermittent and difficult to locate when it is working correctly. I'm just not familiar with a GM brake system to know if there is any component that has that potential. Do any systems come with residual valves built in? A faulty one of those could have potential to hold a pressurized charge after the pedal is released I would think, but that still would not address the near lack of pressurized fluid when the brake is depressed immediately after the charge is released. I still think we're all overlooking something here...
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Old 07-14-2009, 11:49 PM
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Spika,

I believe there is a bit of trash in the system near the master cylinder. On chrome units I have seen them flake off small bits of chrome that will hang things up then move and allow correct operation. I know it is a new cylinder but take it apart and clean it up then blow it off. Use no rags. They leave lint that will do the same thing.

Scholman
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Old 07-15-2009, 09:44 AM
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Quote:
Originally Posted by spika
I don't mean to sound contrary, gentlemen, but your logic has a peculiar smell.
What I posted is not based on logic but on fact. If the master cylinder piston is unable to fully retract when the pedal is released, there will be residual pressure being applied throughout the system. This can and will cause one or more of the calipers/wheel cylinders to enable the pads/shoes to drag.

If that has a "peculiar" smell, it is your brake pads dragging.

Quote:
Originally Posted by spika
Do any systems come with residual valves built in?
If the master cylinder was originally designed for manual (no power assist) braking system with drum brakes, it may have built in residual valves. This can cause too much residual pressure to be in the disc brake calipers and enable them to not fully retract when the brake pedal has been released.

Look in the opening where the brake lines connect to the master cylinder. If there is a residual valve in place you will be able to easily see it. It can be removed and the master cylinder can then be utilized. Another "fix" would be to replace the master for one that is designed for both power brake system and for disc/drum application.
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Old 07-15-2009, 10:20 AM
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My brother is a life long mechanic and I sent him this thread. The reponse he sent me prompted me to re-read the problem and it is so obvious I can't believe it. It IS the master cyl! I read it but it went over my head that the ORIG MS was reinstalled and it worked fine! SOOOOO, JUST the orig master cyl fixed the problem!

HOWEVER now that hoses have been changed, there could be more as he added.

These master cylinders are Chinese junk, I had one that gave me fits, the bore was not round and sometimes the piston/plunger would hang up in the distorted bore of the master. Also replace the brakes hoses, they may look good on the outside, the hole inside is only 1/8" and have been know to close up.. New parts does not always mean good. .. Russ

There you go, and if you go back and read it again, it makes total sense.

Brian
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Old 07-17-2009, 02:49 PM
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Brake woes

As a genral rule there should be a 1/4 inch play between the master cylinder and the rod of a vacuum booster make sure you discharge any remaiming vacuum in the system.Make sure that you dont have any pedal load on the booster check out mbm brakes install video on there web site. It could be that your spacers were not enough. Dont rule out a bad master check the depth of the radius on the piston were the rod mounts to the flange and compair it to the master that work inports have a large tolerances resulting in poor proformance.ie junk

side notes: when the vacuum booster is charged with vacuum or the car is running it tends to move the rod forward to its operating position after vacuum is discharged the internal spring will return it back to bottom out.
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Old 07-17-2009, 03:32 PM
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Quote:
Originally Posted by Tallon
As a genral rule there should be a 1/4 inch play between the master cylinder and the rod of a vacuum booster make sure you discharge any remaiming vacuum in the system.Make sure that you dont have any pedal load on the booster check out mbm brakes install video on there web site. It could be that your spacers were not enough. Dont rule out a bad master check the depth of the radius on the piston were the rod mounts to the flange and compair it to the master that work inports have a large tolerances resulting in poor proformance.ie junk

side notes: when the vacuum booster is charged with vacuum or the car is running it tends to move the rod forward to its operating position after vacuum is discharged the internal spring will return it back to bottom out.
1/4" of play between the pushrod hole in the master cylinder and pushrod coming out of the booster?

I don't think so. .010"-.030" MAX!

1/4" of freeplay and you are throwing 20% of the available travel away, travel you might need if you lose the brakes on one end of the car.



If the booster pulls the diaphram froward at all without any input pressure from the brake pedal it is not functioning correctly.

The tool to check the clearance is easy to make and use, a simple piece of sheetmetal with a moveable depth plunger. Make the plunger the same length as the sheet metal "H" and attach it with a couple of zip ties. Radius one end to match your pushrod . Check the depth of the bore in the piston using the radiused end, then set the other end of the tool on the face of the booster. You can view the clearance at the pushrod and adjust as needed..

Later, mikey
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Old 07-17-2009, 07:13 PM
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Thats a great tool idea and anyone changing componits should make one.I would suggest that the probe end have a radius on it. The fact of 1/4 inch comes from MBM brakes in there install video check it out.MBM is a aproved Chevy restoration vender for vacuum boosters.
from all indications the master cylinder rod is too long baring the master isnt bad.
I build hydro boost braking systems and have just a little experence with this.
In the early stages of prototyping this was(master cylinder rods) a troubling factor for us.
It is not a malfunction of the booster if it moves the rod forward when vacuum is applied it is a forward assist unit to a machanical link. The same applies to a hydraulic system. It will find its own home position when engery is applied.
Of course just keep shiming the master out until until the system works or pedal travel is sloppy. I do think how ever if the masters are measured and they are the same the master is bad. One more thing the radius on the masters should be farily close.
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Old 07-20-2009, 08:03 PM
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Lots of good input...thanks to all! Its interesting how something relatively simple like a hydraulic brake system can have so many not-so-evident facets. Schoman and MartinSR, I think you win the prize as far as I can tell, although all of the posts are important. I replaced the master cylinder with a $30 O'Reilly's cast unit, and all the issues vanished. Using the same booster, same pedal to booster linkage rod, same geometry, same push rod clearnance, etc. I think the chrome unit, like you said, looks good but is crap. I guess I'd rather look a little less shiney and be able to stop in time than the other way around.
Just to address some of the others...yes, this was a front-disc rear drum system in the original donor vehicle, and did work fine for 4-5 years after it's current installation. I hesitated to think that a new MC would be the culprit, but did have my suspicions; although I did believe that a sticky proportioner could result in similar symptoms. Still not quite sure why the uneven wear on the last front pads...its obvious that the passenger side was not retracting as far as it should, and it was that side that was dragging, even after the new calipers. Several miles on the new cast MC now, though, and it has not indicated any drag at all since. I'm convinced the chrome MC was not always allowing the fluid to return. I'm going to periodically check the wear on the pads and see if these new calipers do the same...I guess it is possible that the castings the slide pins thread into are slighty out of line and cause the pad to not slide out as it should. That seems like a long shot, but everything else on that end is new, including the hose, just in case it is separated on the inside and acting as a "check valve".
At any rate, the brakes work as they should now; I've learned to stay away from Chinese Ebay parts unless they're for show only. I'm sure there is a quality US made chromed unit available...just need to look harder and probably shell out a few more bucks. Thats cheaper than finding a new grill for a 46 studebaker...
Thanks again to all-
tjs
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