'74 TCS brake issues - what is failure mode of Master Cylinder?

[analogmike]

Hi,

I had a thread last year about my '74 brakes, the rears were not working much.

I got new shoes, adjusted them well, still very weak. Slave Cylinders are like new. Pedal is firm.

Then I got a new rear booster and after 10 hours of bleeding with pressure bleeder, vacuum bleeder, and pumping the pedal, the brakes work, but rears are still very very weak.

I was thinking that maybe a bad master cylinder is what caused the bleeding to take so long, and is causing the weak rears. Is it possible that the rear circuit in the MC (gasket/o-ring) is leaking, (into the front?) and the front circuit still working fine? Have not had this issue on any of my dozens of classic cars.

[jbcollier]

It's possible.

Try this first:

Apply the brakes hard on a gravel surface.  Do the rears lock?

Myself, I would remove the boosters and fit an 0.70 master cylinder.

[EuropaTC]

I was thinking that maybe a bad master cylinder is what caused the bleeding to take so long, and is causing the weak rears. Is it possible that the rear circuit in the MC (gasket/o-ring) is leaking, (into the front?) and the front circuit still working fine? Have not had this issue on any of my dozens of classic cars.

Hi Mike,

As always with an old Lotus anything is possible but from the rest of your post it doesn't sound as if the m/cyl is at fault with the brake pedal being solid. 

If the front or rear compartments of the m/cyl were leaking pressure into the other, then in effect you'd have a single circuit m/cyl and both brake  circuits would still see the same pedal pressure.  So as far as the braking circuits were concerned, with no external leaks they'd get the same pressure.

Ok, it's potentially controversial but my view is that the rear drum brakes aren't that great anyway, so you might already have the best you're going to get. Richard Hill of the Yahoo group produced a spreadsheet of the braking process which is a good representation of real life where it's quite hard (impossible ?) to lock the rear drums because the front discs take up so much of the braking force.  If you want to see if they'll lock at all, you might have more luck just using the handbrake or trying the brakes on gravel where the fronts lock instantly and give the rears a fighting chance.

[4129R]

You can do the test slower and safer on wet grass.

[jbcollier]

Unfortunately no.  You can have no external leaks and no pressure in one or both systems.  The m/c can bypass internally.  Usually just one circuit but VW beetles were famous for both going at the same time.  If both go, then your pedal goes straight to the floor.  If one goes then how much extra travel depends on which circuit.  With the front bypassing, the pedal will be quite low but the pedal may feel almost normal if the rear is bypassing.

[analogmike]

Thank you for the replies!

The rear brakes DO work, they get quite hot when using them, they are just super weak. On my gravel driveway I can barely get the rears to lock, the fronts destroy my driveway before the rears lock.

I actually have a few other things working against my poor drum brakes that I did not mention...

1) The front booster is a higher ratio, 2.3 vs 1.9 for rears. Maybe they should be the same or I could even swap them (or idea below).
2) Front calipers are GT6, a bit more powerful.

One idea is to send the front booster output to the rear brakes and vice-versa, just need to make up two extension/adaptor lines. This would give me some good info to figure out what's up. Wish I had a pressure gauge for brakes, that would accomplish the same thing easier, maybe I can borrow one from my race shop.

Thanks again!

[EuropaTC]

Hi Mike,

Ahh, that's interesting to hear (and it sounds like we both have gravel driveways    )

I don't know enough about boosters/servos to comment sensibly, I would have thought they should be the same ratio but in the UK we only ever had the one so I'll wait for someone from your side of the pond chip in there.

Thinking about what you've got and comparing it to the calcs I've done on my system, I think you've now got a greater imbalance between front & rear and your comments comparing front/rear brakes are pretty much what I'd expect to see.

In standard form the TC braking was very roughly   75% front / 25% Rear.    By increasing the front calipers to the type 16/GT6 type, and presumably the discs from 232 to 245mm as well you've now got something like 79/21, which makes it even more likely that the fronts will lock before the rear. 

I'd imaging having different booster ratios will increase this further because the front system pressure will be fractionally higher and hence greater clamping force, but I'd need plenty of thinking time to work it out.

Apart from the servo ratios I'm not sure you've actually got anything wrong, I think the system is behaving as you'd expect from the calcs - front lock up and the rears never getting anywhere near to lock-up. (unless the fronts slide for long enough  ).  Swapping the front/rear booster as you mention should improve things but it's a pure guess as to how much.

[jbcollier]

Common mistake to "improve" the front brakes and throw the brake balance way off.  For the street, the stock brakes work just fine.  Highest braking force ever recorded when new.  I fit green-stuff pads and left it at that.  My brakes are great with a somewhat higher pedal force required than what we are used to with modern cars.  This is good as it makes modulation easier.

So, if yours is a street car, unimproved it.  Fit the stock calipers, either remove or match the boosters and use a 0.7" m/c if you remove the boosters (a good idea).  If it is a track car, then still remove the boosters and fit rear brakes that better match the new fronts.

[cwtech]

All above info is good, but let's get back to the basics.

How much of the rear shoes is making contact with the drums?

New shoes? - Yes.  ....Adjusted properly? - yes.

But, remember when brake shoes had to be ground to match the radius of the drum?  ....New shoes do not always match the drum.

I've encountered this on several vehicles (non-Lotus).  ....An easy test is to apply chalk lines to the brake shoes.  ....Drive the car a short distance or run it on jackstands.  .....Apply the brakes.   .....Remove the drums to see how much of the chalk lines remains on the shoes. 

[analogmike]

Good info on the shoes. I did notice very little contact initially, so I sanded the shoes down to make them fit better.

I'll try to make up some adaptor lines to run the boosters to the opposite end of the car, it might just be the perfect setup or at least it will give me more data.