oddball

Two topics, one thread.


Any hints on alternate master cylinders? A rusty cast iron master just really bugs me, and I haven't had any luck with the "master cylinder paint" lasting long. Looking for disc/drum as well as disc/disc.(*)
My primary quirk is I prefer something with a usable cap for pressure bleeding. The flat bleeding plates never seal very well.



My current system is a bit of a hodge-podge, all components are new
72 Cutlass Supreme, 403 with TH350, 14" wheels

Hydroboost (14" vacuum @ idle, pretty marginal for a vacuum booster)
Mid-80's Dodge D150 master (aluminum, disc/drum, no hold-off valve)
Stock combination valve
Stock front discs
Adjustable proportioning valve and 10psi residual valve in the main line to the rear brakes after the combination valve
Stock rear drums

This setup has terrible front/rear balance, sadly. I have to keep the adjustable prop valve turned all the way to minimum pressure to prevent the rear brakes from locking up first - even under moderate braking. It'll lock up the fronts, so I don't think anything is wrong on that end.

Not sure what went wrong there. I've heard of this combination in fairly wide use in the Chevelle world. It's perfectly driveable with the adjustable prop valve, I just worry I'm leaving some braking performance on the table.

I've actually disassembled this master cylinder for inspection since I've gotten 4 or 5 that had significant scoring on the cylinders. Gotta love parts suppliers.


(*) Are disc/drum and disc/disc masters really different? This digs into some master cylinder design theory. The aftermarket units are very unclear on this point, some saying they can drive any system in the world. In the end, it's that disc and drum need different volumes of fluid, right? However, rear disc pistons are consistently significantly smaller than front pistons, although probably still larger than drum cylinders. There's residual valves for drums that discs specifically don't want. Most of the master cylinders I've taken a close look at have separate pistons that are balanced by springs. This implies they can move different amounts to move different volumes of fluid with the springs balancing the pressure delivered.

joe_padavano

The only thing the brakes care about is M/C bore diameter and available stroke. The only functional differences between drum and disk M/Cs is the use of a residual pressure valve built into the outlet ports of the drum M/C (at least for GM cars, I can't comment on non-GM masters).

The use of a larger reservoir for the disk side of disk/drum M/Cs is for convenience only (so you don't have to check fluid level as often). There is no functional benefit to the larger reservoir as far as braking performance is concerned (that is, unless you run out of fluid in your calipers, of course ).

The number one requirement is to match the M/C bore diameter with the wheel cylinders. If you have factory brakes, go with the factory M/C diameter. Drum brakes need the residual pressure valve. If you use a disk/disk M/C (like the aftermarket "Corvette" M/Cs that are available), run a 10 psi valve in the rear brake circuit.

Do you have an adjustable prop valve AND a combo valve? You shouldn't. Use only one or the other, not both.

oddball

Thanks, Joe. The problem I have is the rear wheels easily lock up when just using a stock style combo valve. I added the adjustable prop valve [/i]between[/i] the combo valve and the residual valve to further limit the rear pressure. I've been ordering disc/drum combo valves, hopefully they do actually have the proportioning valve in them! Can't say I've been impressed with the quality so far.
IIRC, this master as a 1 1/4" bore, whereas stock was 1 1/8". I also moved the brake rod from the power to the manual hole on the pedal rod. Using the power hole resulted in incredibly sensitive brakes with very short travel.

joe_padavano

Edit: Sorry, I missed the "hydroboost" - revised text below.

Your master is too large. Using such a large bore with the manual brake pedal ratio dramatically increases the amount of fluid pushed per inch of pedal travel, thus the overly sensitive brakes. Also, a large M/C with manual brake pedal ratio results in much lower line pressure for a given pedal pressure. This likely requires you to press harder on the pedal to get sufficient pressure in the calipers, but over-pressurizes the self-energizing drums. I also suspect that the hydroboost is more sensitive than vacuum boosters. I would suggest a 1" bore M/C and the power brake pedal ratio; certainly no larger than the stock 1.125" master. You likely will want the smaller master if you go with four wheel disks. Use only one prop valve, either the combo or the adjustable.

As for paint on an iron M/C, Eastwood now has master cylinder paint that is supposed to be resistant to brake fluid. I bought a can but haven't tried it yet. Naturally, you have to ensure that the surfaces on the M/C are COMPLETELY clean prior to applying the paint. A good scrub with lacquer thinner is a minimum prior to painting.

oddball

Hang on. Which hole is which on the brake pedal? I thought the one closer to the pivot (higher on the pedal) is the manual hole since that has more leverage.

joe_padavano

That is correct.