356 Brake Problem

[Winston Teague]

Dear All, i have a snag:
The rear brakes are more effective than the fronts.
Car '61 T5B Coupe.
When pushed hard the back brakes will lock before the fronts, in fact the fronts don't lock on dry tarmac. The car doesn't dive under heavy, straight, level braking. This situation is hairy in an emergency stop. So far i have adjusted all. No change in performance.
Then i replaced the front shoes with PRS exchange linings, (thanks chaps for the excellent service), no significant improvement. I then looked at the rears, quite worn, so put the really very good ex front shoes on the back, and will return the ex rears to PRS as exchange units.
After a short test run with 8-10 vigorous brake tests, all drums are at the same temperature, though the rear right is the most eager to lock up.
I appear to have a dual circuit single master cylinder, with dual reservoirs. This implies to me that the two circuits are completely separate. Could it be that the well adjusted rear brakes are preventing the fronts from receiving enough of the pedal pressure? Surely there must be some 'compensation' between the two circuits, otherwise it would be impossible (?) to bleed? Am going to try bleeding the fronts tomorrow.
Any suggestions?
W

[Mike Smith]

Ring me Winston, it is easier for me than typing acres of script

01268 711 142

Mike

[POR68]

Winston, there is a long thread on split system master cylinders on the US 'site that you might like to read. I contemplated adding one of these but held off as I have a new single stage MC that I fitted about 2 years ago. It works perfectly. I also replaced all the linings on my 'shoes, with a local brake place bonding on new shoe linings for all four wheels. I carefully numbered each drum and its respective shoe and position (trailing/leading, etc). I asked the brake shop to 'radius' each new lining exactly to the drum for that wheel. The whole process was cheap.

If your linings are of unequal hardness there can be problems. If they are not making equal contact there can be problems. If the flexible brake lines are not all in good condition there can be problems. If the fluid pressure to the wheel cylinders is therefore not balanced ... etc. Swapping shoes around is a bit of a lucky dip. It is good to inspect the shoe linings to see what pattern of contact and wear they are showing. This can give you a clue as to their effective fitment. Your rear shoes will probably have good contact patterns but as you say they are worn and may need replacement anyway.

What do your metal bonded brake drum linings look like? Are they , smooth, scored or cracked? Is the rear handbrake adjustment dragging?

There are many things this could be (including the split system MC not performing correctly) but as you say it is dangerous, especially in any wet conditions. Do bleed the front wheel brakes then readjust them so they are just dragging slightly. Do the same on the rear and perhaps back off the rears a bit more than the fronts and try that. This is not the optimal fix however, but a temporary test only.

I would be fitting radiused shoes on all 4 drums and starting from that position with a full flush of the system with new fluid.

[Winston Teague]

Thanks for the views and help so far chaps, bleeding the fronts pumped rather murky fluid through so i kept going till it flowed clean, and locked off. Some improvement. Mike has been very helpful and we both suspect a master cylinder problem with the front circuit, a theory supported by the dirty (rust?) fluid. A chat with Paul @ PRS next......
W

[POR68]

You're in good hands there Winston. Might be time to replace the old shoes with ones of similar hardness to the ones you have done already.

[Winston Teague]

Good hands indeed. Wise and very interesting words from Mike got me somewhere, then a chat with Paul at PRS followed. Many thanks, both, for your time and help. PRS sent me a new master cylinder as we could not brainstorm any other possible sources for the problem. As ever PRS were prompt and inexpensive. Master cylinder swap was straight forward as the new one is indentical, a session with a Gunson Easybleed and running lots of new fluid through the whole system and i now have much better brakes.
The rears are still eager, a tendancy that i hope to resolve or at least improve with some time on the high spots on the new front shoes, and by playing a bit with the adjustment of the front and rear shoes, perhaps allowing a bigger air gap between the shoe and drum at the back......
W

[POR68]

Winston can you remove the front shoes and drums and take them to a brake place to have them radiused to match each other? They will not do so otherwise as the drums tend to 'evolve' over time. Do the rears at the same time and I would replace them also with shoes of the same hardness as the front. You will be very impressed and it is not costly.