Soft pedal

[jvandyke]

My brake pedal has suddenly gotten very soft (has always been a bit soft for my liking).
Now I can push it till I feel some sort of stop, still have brakes but obviously that's not right. Can pump it up a bit but it doesn't last.
Is this a master cylinder going bad? No fluid loss.
I don't see much in the trouble shooting guides in the FSM and such about soft pedals, just hard.
Also, my brake booster vacuum hose enters the booster chamber right on the side, not the front as pictured in the FSM. Not sure if this tells me anything, just an observation.
Started hunting for rebuilt MC, hopefully they can be had yet. I've rebuilt a few before but for the marginal price difference, I think I'll get a reman one.

[Anonymous D]

It could be a failing master cylinder, or a few other things too.

My suggestion would be to do some system maintenance:

Inspect front pads, front rotors, and rear shoes and drums (verify surfaces are flat)
Check inside the booster, for fluid (indicates a leak at the M/C front seal)
Bleed the brakes (to eliminate possible air in system)
Adjust rear shoe eccentrics (this can make a difference)
Adjust parking brake cable, to full engagement at 8 clicks up (this helps adjust shoes too)

Then monitor fluid usage (and loss) at the master cylinder

[namba209 (R.I.P.)]

A. Dave is right about the rear brakes, the front discs are self adjusting. All the rear brake adjusting is done by the two rear eccentrics at each brake, then adjust the emergency brake to finish off the job. To check for an internal bypassing MC, put a small amount of pressure on the brake pedal. If the pedal goes down, then the MC needs to be replaced or rebuilt. HTH.

[jvandyke]

I went through the brakes last year. Rebuilt one front caliper, other seemed good, new rubber hoses, new rear cylinders, new pads up front, adjusted rear shoes and e-brake and of course bleed the daylights out of it. Since the level of fluid has not changed and I see no evidence of leaking I pegged the MC as the culprit.
Not impossible I've had some air in the system the whole time and now it's moved somewhere where it's more evident but I sorta doubt it. Would this symptom be indicative of a leaking seal on one of the MC's piston's seals? Letting fluid move past the piston instead of squeezing it?

[markandson]

I went through the brakes last year. Rebuilt one front caliper, other seemed good, new rubber hoses, new rear cylinders, new pads up front, adjusted rear shoes and e-brake and of course bleed the daylights out of it. Since the level of fluid has not changed and I see no evidence of leaking I pegged the MC as the culprit.
Not impossible I've had some air in the system the whole time and now it's moved somewhere where it's more evident but I sorta doubt it. Would this symptom be indicative of a leaking seal on one of the MC's piston's seals? Letting fluid move past the piston instead of squeezing it?

Yes, if a seal goes bad in the MC it will allow fluid to pass from the pressure side of the piston across the seal to the reservoir side giving you poor brake performance and a squishy feel of the pedal. If your front disc seals are going bad and they are stuck to the pistons it is possible that they are not letting the piston slide through and self adjust in which case they would act like springs and pull the piston away from the rotor when you release the brake pedal. This isn't too likely but something to think about if you have not rebuilt both of your front calipers.

[West Coast GT]

A number of us have had no luck locating a new or re-manufactured master cylinder this last year or two. I've found them listed on-line through national parts houses (as recommended by others on this forum) but found availability to be "Out of Stock".

Last time I checked OGTS was approx this time last year. Remanufactured MCs were on back-order.

My recommendation is to order the re-build kit from OGTS if they are still out of stock, and carefully disassemble your MC. Do not scratch the cylinder bore or you will be screwed.

I've found that using compressed air to remove the shaft & seals from the bore worked the best for me. (Although I shot the entire assembly out of the bore into the garage ceiling and almost nailed the hood on my wife's Miata the 1st time - not to mention misting the garage with a fine spray of brake fluid)

[jvandyke]

I've called a few places, they're checking availability. Is there a difference between the rebuild kit OGTS sells and the ones I see listed? (besides a substantial price difference?) Darn, right when the weather turns good and the car is running great, it decides not to stop great.
I could plug the outlet ports and see how the pedal feels but what to plug with? Standard bleeders work on the MC outlets?
I remember JoAnna had trouble finding one last fall too.

So far, no one can get. Rockauto will have mine rebuilt for the cost of a reman one, $32. That's cheaper than a kit and trying to do it yourself but you have to have 2-3 weeks to kill.

PS
Just found this thread, sounds like I'm on the right track. Didn't come up in a search because, although it's all about MC it never used the words Master or Cylinder!

[West Coast GT]

I'm only a shade-tree mechanic, tinkerer really, but I'd have someone step on the brake pedal (with the engine off) and listen for intense swishing sounds in the MC. If you don't have a mechanic's stethoscope, I've found a long rachet extension or extra-long funnel work well.

You'll be listening for the sound of hydraulic fluid getting squeezed back past the seals (from the pressurized area to the non-pressurized area).

[jvandyke]

I'm only a shade-tree mechanic, tinkerer really, but I'd have someone step on the brake pedal (with the engine off) and listen for intense swishing sounds in the MC. If you don't have a mechanic's stethoscope, I've found a long rachet extension or extra-long funnel work well.

You'll be listening for the sound of hydraulic fluid getting squeezed back past the seals (from the pressurized area to the non-pressurized area).

Good idea. As a fellow tinkerer, I've since discovered you can remove the lines of the MC, put in plugs, then see what the pedal does. If it's still squishy it's the MC internals. If not, the problem lies somewhere outside the MC.
I'm sure it's my MC though, explains pedal issues all last year and now complete failure. Looks like I'm rebuilding it myself as no one can get their hands on a rebuilt.

[guyopel]

The M/C kit you get from OGTS has seals that do not come in other Master Cyl. O.H. kits and has the upgraded rear piston stop and seal . HTH
John

[jvandyke]

The M/C kit you get from OGTS has seals that do not come in other Master Cyl. O.H. kits and has the upgraded rear piston stop and seal . HTH
John

Oh well. I already ordered one from Autozone at $22. Has seals and pistons it appears. Hopefully it gets the job done. Apparently no ready-to-go rebuilds on the market still.

[Anonymous D]

My recommendation is, to also check your reservoir seals (if very worn, air can get sucked in there) and that a rebuild kit that comes with well-written instructions, can save you time and expense (beyond the immediate savings of buying on price alone).

In recent weeks alone, persons rebuilding GT master cylinders have: Introduced air into their systems (and wrongly blamed the rebuilt m/c), stripped threads to output hoses, snapped the plastic "elbow" connection -- and in another case a hired mechanic ruined paint on top of one headlight (by dripping brake fluid onto it). All these complications are preventable, had the owner acquired and followed GT-specific master cylinder rebuild instructions.

[sawdust]

Oh well. I already ordered one from Autozone.

Let us know the outcome of this one from Autozone.

[jvandyke]

Let us know the outcome of this one from Autozone.

Of course.
And the pitfalls mentioned by AnonymousD are welcome reminders as well. Most of which I'd have to deal with even if I were just going to pull the old and replace with a reman. As to the actual rebuild, I'd love a good set of instructions but by the time these got to me, I'd have long since done it myself anyway.
I'd suggest that whoever runs the club make these babies digital downloads at $1 a pop. I'd have long ago bought quite a few. Lots of time to scan 'em properly though I'm sure, it would probably pay off long term.
MC rebuild: Not too hard a job I don't think. Obviously I'll test extensively before I trust my life to my work but that's what big parking lots are for. Last year after I went through it all, first stop was a lot down the street and several successive panic stops, if it were to fail, that's when I'd want to find out.
I remember I got one of the seals in my MG Midget's in wrong. It leaked all over my foot so that one let me know!

[jvandyke]

Promised to report back.
Rebuild kit was good, all the right parts it seemed and pistons too.
Thread and pictures are here.
Got 100+ miles on the GT since and brakes feel great!
At least that's one thing lately that went well.
So I guess I can recommend that rebuild kit. Price is great, parts are correct and since rebuilt M/C can't be had at the moment, do it yourself I guess.