Brake system failure

[bamabucket]

We were preparing for a Sunday drive around noon today but had to cancel it when we had a total brake failure. The brakes were OK when I started the car and drove it out of the garage but when I stopped in the driveway the pedal went to the floor and I had no braking whatsoever. I don't have an external return spring but can pull the pedal up. When the pedal is depressed and returned by hand, I can feel quite a bit of resistance, but not the mushy feel I would expect with air in the system. I have done an external check for leaks and don't see any from any part of the system and there was fluid in both of the chambers of the master cylinder. I will check the wheel cylinders later but can't see any fluid that has leaked down and out between the drums and backing plates. In any case, a leak in a brake line or wheel cylinder shouldn't result in a complete failure because of the design of the dual master cylinder. My system is very basic with 4-wheel drums, no power assist, no proportioning valve, and no residual pressure valves. It looks like the most likely problem is a master cylinder failure. The master cylinder was replaced 5 years ago with the low-end generic unit Speedway sells. What I don't understand is how the master cylinder can fail completely with no evidence of leaks.

I haven't experienced this type of failure since the days of single chamber masters and that is why I have made this post to see if others have had similar experiences and can explain how this can happen.

The good part of the incident is that in happened in the driveway instead of in traffic and replacing the master cylinder is a very easy job.

 

[T-Test]

We were preparing for a Sunday drive around noon today but had to cancel it when we had a total brake failure. The brakes were OK when I started the car and drove it out of the garage but when I stopped in the driveway the pedal went to the floor and I had no braking whatsoever. I don't have an external return spring but can pull the pedal up. When the pedal is depressed and returned by hand, I can feel quite a bit of resistance, but not the mushy feel I would expect with air in the system. I have done an external check for leaks and don't see any from any part of the system and there was fluid in both of the chambers of the master cylinder. I will check the wheel cylinders later but can't see any fluid that has leaked down and out between the drums and backing plates. In any case, a leak in a brake line or wheel cylinder shouldn't result in a complete failure because of the design of the dual master cylinder. My system is very basic with 4-wheel drums, no power assist, no proportioning valve, and no residual pressure valves. It looks like the most likely problem is a master cylinder failure. The master cylinder was replaced 5 years ago with the low-end generic unit Speedway sells. What I don't understand is how the master cylinder can fail completely with no evidence of leaks.

I haven't experienced this type of failure since the days of single chamber masters and that is why I have made this post to see if others have had similar experiences and can explain how this can happen.

The good part of the incident is that in happened in the driveway instead of in traffic and replacing the master cylinder is a very easy job.

Sounds as if you have a piece of dirt/sand in the M/C causing it to bypass the plunger/cups. Wheel cylinder/1 or 2 wouldn't cause this unless it was one in front and one in rear with a dual M/C. Good luck and get it fixed soon!

 

[bamabucket]

Thanks Bruce, I didn't think of that and it seems logical because it also accounts for the resistance to moving the cylinder pushrod. I agree it is probably not a wheel cylinder or a leak in the lines to the front or back wheels because it would be very unlikely to have a simultaneous failure on both a front and back component. I will definitely get it fixed soon so we can safely get back out on the road. I intend to get a new master and will take the defective one apart to see if there is dirt or sand in it.

 

[iroct]

check for mechanical failure in pedal or linkage to master cyl.

 

[2old2fast]

Having lost brakes in a very bad place once , glad it happened where it did ...gonna be interesting to find what cause it..
dave

 

[RPM]

Sounds like a bad master to me.

 

[bamabucket]

I did check the pedal arm, cylinder pushrod, and clevis - all OK. It has to be a bad master, but I have never had a complete failure like this with a 2-chanber unit and I will disassemble the bad one to see if I can figure out what happened. I may replace it with one like it from Speedway, but it should have lasted longer than 5 years. Dave, you are so right about where it failed being a good place - I hope your brake failure didn't result in injury or damage. I am glad I have a parking/emergency brake in case something like this happens again. I am not going to get to repairing it for a few days and I'll post the results of my examination of the defective unit then.

Thanks for the responses. I appreciate the opportunity to get help from fellow bucketheads and hope this site will continue. Thank you Ron and Spirit for stepping up to keep it going - I'll be happy to make a contribution if and when needed.
Lou

 

[Hotrod46]

One other thing to check is that you have enough travel in the pedal to stroke the MC all the way. If one end fails the piston has to travel far enough to mechanically contact the second piston. If it doesn't, you won't have any brakes with a failure even with a dual circuit MC.

Push the MC all the way in on the bench and measure the stroke. Your pedal needs to push that far. Just to make sure, after the system is bled, crack a bleeder on one end and see if you still have some pedal.

Mike

 

[roadmonster]

To check your master cylinder, pump it until you get a hard pedal and then hold it. If it slowly bleeds down, you have a bad m/c. I had repeated loss of pedal after replacing my brake light pressure switch. It took several vacuum bleeds and lots of fluid to get all the bubbles out. The reason is the low, frame-mounted m/c. A firewall mount allows the bubbles to migrate out through the reservoir, but the T reservoir is not the highest point, so bubbles can be hard to purge. On a T, no pedal does not necessarily mean a bad m/c.

 

[2old2fast]

Hey Lou , did you get your brakes fixed ??

 

[bamabucket]

I haven't had the time to get started yet- too many other things to do but hope to get at it soon.

 

[bamabucket]

I finally got around to working on my brakes today, There were no problems with the mechanical linkage to the cylinder which was full of brake fluid in both chambers and didn't show any leakage. I took it out and bench tested it by using the pushrod to try to pump fluid out of the line ports. The cylinder can be pushed in and returns but no fluid came out of either line port, confirming the total failure condition when in the car. I drained the fluid from the bowls and there was sludge in the both of them that looked rusty, indicating water was in the system (hopefully just in the master). I removed the pistons and the seals looked to be in fairly good condition so I am guessing the sludge had blocked the ports in the bottom of the bowls. I didn't feel like testing that theory by cleaning and testing and am going to junk the cylinder.

The master is a cheap Speedway universal cast iron model that I installed less than 5 years ago to replace a failed master of the same type (age unknown). I used the same cylinder then because I was getting ready for our first trip to a national meet and wanted something that I could quickly just bolt in. I don't want to have the same problem again so I am going to get a higher quality cylinder this time. Speedway sells a Corvette type cylinder for about $98 that is aluminum with a stainless steel bore sleeve that might last longer, but there are many other units available that may be better. I have all drum brakes and am planning to put front disks on later. As I understand it the Corvette 1" bore cylinder would work for my present system now and also for disk/drums in the future.

I would like to get advice on selecting a cylinder and suggestions on preventing water getting in the system. The master is under the floor, so it is exposed to water and other debris but it looks like the same setup many T-Buckets use. I dodged a bullet for this failure because it happened in my driveway so I want to minimize the risk of a potentially serious problem on the road. I am not looking for the cheapest solution, just the safest. I intend to completely re-plumb the system later with new hard lines and braided hoses but want to start with a good master cylinder.

 

[EX JUNK]

If you are using regular brake fluid try switching to DOT 5 silicone as it does not absorb moisture like the old DOT 3 & DOT4.

Jim

 

[bamabucket]

Thanks Jim. Excellent suggestion that I will implement when I get the new cylinder installed.

 

[409T]

You need to clean out all the DOT 3 or 4 before adding DOT 5. If you mix the two you the fluids will turn to jelly, maybe what you have right now.

 

[bamabucket]

I did some searching on the net about DOT 5 and found postings with information on how to thoroughly flush all traces of DOT 3 or 4 from the system and how important it was to dot it so it won't turn to jelly. The only brake fluid I have on hand is DOT 3. This incident has reminded me that it is import the periodically flush brake systems using DOT 3 or 4 , which I have neglected to do.