Master cylinder push rod named full in, no brakes

[joegyoung]

I am rebuilding my master cylinder on TC 3443 after the brakes failed yesterday afternoon. Turns out that the pushrod got jammed into the MC and could not return so you could not apply any pressure as the piston was left at the end of its stroke. After taking it to pieces I found that the MC was a bronze after market and the push rod had gotten hung on the ears of the circlip. I have included a picture of the clip so that you can see the bent ears or holes used to insert it. Has anyone else had this problem with the MC push rod? It took 15 or 20 minutes after I got the MC off the car to get the push rod free and the MC components apart. I am pondering how to get it back together and not have this repeat. Part of the issue was low brake fluid but not all of it as the rod was much more chewed up then just this one hang up.

[Duncan M]

Bronze inside the bore? After getting honed out enough times, rather than replace the whole thing you can have it bronze sleeved. So it might well be an original that has been sleeved. Sounds like yours got the piston shoved in too far. Could be adjustment. A lot of TC's have a steep angle from that ball joint at end of piston (outside the rubber boot). That steep angle is not good. Sometimes you can lessen it quite a bit by loosening the big bolt at the other end where it mounts, as there is an oval and not round hole there. For final adjustment, there should be a little play to allow the piston to go back to its correct position. There are two small holes in the top of the bore, one larger than the other. I am not sure of the technical term, but if the piston goes past that farther hole you are in trouble. It you hit the circlip you definitely went past that hole.

[Steve Simmons]

Phil Marino used to make solid bronze master cylinders, I believe with tomato soup cans for reservoirs. It could be one of those. Duncan is right, you need to make the push rod as flat as possible, and ensure it isn't too long. There have been different push rods and pistons made over the years as replacements so if you have a combination that pushes the piston in too far then it can get stuck. Not good!

[Eric Worpe]

Comments about the steep angle of the ball joint are "spot on". This can also be corrected if the bracket supporting the master cylinder is twisted back to its correct position. The bracket tends to twist due to the down-wards thrust of the push-rod and consequently exacerbates the situation due to the increasing angle of the ball joint. I used a length of 1.25" dia. pipe with a nut welded inside to take a 3/4" bolt that clamps the bracket through its oval hole. This should ensure the bracket can be twisted without distorting the flat surface of the bracket that supports the master cylinder. Use beefy washers either side of the bracket when tightening the bolt.

[frenchblatter]

Am I missing something here. The pushrod went in so far it jammed in the circlip. The pushrod shouldn't go that far in. There was something else wrong, maybe a fluid leak, that cause the pushrod to go that far in.

I don't think you have a problem with this, stop the pedal going to the floor, pushing the rod too far in a and the pushrod problem is solved.

Or, as I said, have I missed something, as usual.

[Eric Worpe]

Hi Norman, you've not missed anything as I've jumped in without an explanation. I think what's happened is due to the excessive side thrust of the push rod. This has forced the push rod to become jammed in the master cylinder as the circlip has blocked the return of the push rod's elbow. You are quite correct in stating the push rod must have gone in too far due to lack of adjustment or a leak. It could be that the excessive side thrust is inevitable when the push rod goes in too far, but it's not helped when the elbow angle starts out as too great.

[frenchblatter]

Eric, fix the leak and adjust the shoes and see how much travel the pedal has. There are a few other master cylinders I've seen that don't like to be pushed in to far due the acute angle created. I think if you fix the underlying problem all will be well.

Good luck.

(I'll be refitting my brakes later in the week. Watch the blog to see how I get on)

[Duncan M]

Improper adjustment between the actuating rod and the plunger can cause this problem if the plunger is not allowed to fully withdraw to its proper starting point. At that proper starting point, there should be just a barely detectable gap between actuating rod and plunger. It there is no gap, or play, then it is possible the plunger is starting from a point where it is already being pushed in some. This could allow the plunger to go past the second hole--causing the problem. Of course if the spring returning the plunger to its starting point is wrong, or if the plunger is not the correct length for the particular M/C, the same problem can occur. The plunger should always start at the same point, and the play between the actuating rod and plunger should only need adjusting when the M/C is removed and replaced. Proper adjustment of the brake shoes, as they wear, is necessary to keep the plunger within its travel limits. A small leak should not cause this problem, unless it is introducing air bubbles into the system. Or pressure bleeding silicone fluid is known to introduce air bubbles; either of which could cause the plunger to go past the second hole.