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Repair/Overhaul of Scarab brakes - Part 1
by Peter Withers Date of article:October 1985 Category / Type:Technical Bevel
The Scarab braking system was fitted to Ducati 750 models along with Lockheed and eventually Brembo.
Owners of machines fitted with the latter will have no problems as spares are still readily available, however for those purists who prefer originality the Scarab presents something of a problem, parts and seals being unobtainable from original sources. If you have put your Scarab on the shelf or are about to here are a few tips which might allow you to retain your system for a few more years
Master Cylinder: Remove lever followed by piston, this pulls straight out and is fitted with 2 seals, next comes the spring and finally at the base of the bore' is a small metering valve fitted with 1 seal. This valve should tap out. Note the flat face of this valve faces inward and if stubborn a blast of air from a compressor or foot pump up the outlet pipe will shift it. Next remove the 6mm Alan head screw on the end of the body with a 3mm Alan key, tap the body on end and a ball bearing will follow. If you have the all metal type master cylinder dismantling is complete, however the plastic bowl variety invariably leaks at the base joint. This can be cured by removal of bowl and smearing the treads with Teflon compound obtainable from Locktite stockists. Some difficulty may be experienced in removing the bowl but the following might be of help:- wrap a cloth around the bowl then rap the ends around a stout bit of wood or flat metal, by twisting the cloth round your wood or metal handle the cloth will grip the bowl enough to allow it to unscrew in the normal manner without damage.
Next examine piston and bore. The bore rarely wears excessively but sometimes a small ring of corrosion about 15mm down will be noted on examples with alloy bores. Brass insert bores do not suffer this defect. Standard pistons have a diameter of 0.620 in. and are still usable down to 0.618 in. but if worn below this seals will not function properly. Okay, if you have a brass bore and a useable piston, lightly hone the bore. This can be done easily: cut a small strip of 6oo grade wet&4ry paper, slot a section of old pencil or dowling, place your wet & dry in the slot , fix the other end of pencil in an electric drill and run the whole lot up and down in the bore until a polished finish is obtained. Next fit new seals. "What new seals!", you say. Well front up to any Patons Brake outlet and purchase seal number P6016A. These seals also have the added advantage of being able to accommodate oversize pistons to 0.625 in. (Note: this seal is also found in British Leyland cars eg Morris, Austin, Rover etc and all the three seals in the Scarab master cylinder are the same).
For those of you who have badly worn pistons and/or the dreaded ring of corrosion do not despair, these can be brought back to operational by machining modified replacement pistons. See diagram
X: Remains the same, but note that the thrust is taken over a larger area negating effects of localized wear in bore. Y: Diameter increased as necessary for snug sliding fit in bore. Maximum allowable size is 0.625 in as the seal will not work on diameters lager than this. Distance X can be increased to compensate for lever wear but if taken to extreme or excess then the brakes will not be released. Too little results in a spongy feel at the lever. This modification is usually enough to fix 90% of units, however if your bore is past this the only alternative is "specialist attention" in the form of a shrink fit liner. Patons Brakes used to do this service a few years back but it was very expensive. It goes without saying that all components must be clean and dry before reassembly and seals face inward eg lip towards the closed end of body. If using normal glycol brake fluid apply red rubber grease to seals during assembly. This is obtainable from any brake/clutch outlet (BGT ,Patons etc), however I recommend using Dow Corning Silicon brake fluid. This has a higher boiling point, does not absorb water, will not ruin paintwork and will keep corrosion at bay while giving a brake system a life many times that of brakes using conventional fluid. A good point if you want your Scarab system to last. When using silicon fluid do NOT use red rubber grease instead soak seals for about 2 hours in the fluid before reassembly and the brake fluid is available from Penrite Oil Brunswick - maybe the club might stock it? When replacing the ball bearing use a new one obtainable from any push bike shop and place in the bore then give a light tap. Before replacing Alan key grub screw, make sure the tiny hole in the body just above is clear as it's vital for bleeding.
