SOHC.co.uk Forums > CB350/400
Weeping brakes
TrickyMicky:
Use of PTFE tape on domestic gas fittings - OK. Max pressure of domestic gas is 60PSI
Use of PTFE tape on domestic mains water - OK. Max pressure of domestic water is 80PSI
Use of PTFE tape or any other sealant on hydraulic vehicle brakes is a no-no! All fittings must be assembled dry. The 'lock-up' pressure in a non-servo assisted brake pipe is approx. 900-1000PSI. The metal brake pipes have to be manufactured to withstand a test pressure of 5000PSI. Leave the PTFE tape in the drawer in the cupboard with the plumbers fittings.
ka-ja:
I agree that hydraulic systems are best built dry of sealants etc., But having spent my working life on control systems and industrial hydraulics operating up to 12,000 psi, loctite sealants were often used, industrial breakdowns costs were to be avoided if at all possible
Bryanj:
As far as im concerned fix the problem not the symptom
AndyD:
Thanks everyone,
Interesting range of views and I guess no 100% simple answer.
I haven't had any brakes in the past with the screw-in flared type connector, always had banjo bolts and washers so I wasn't sure if there was something I missed at assembly.
The bleed nipple weep seems to have stopped with a tiny extra tweak tighter - I'm just nervous of overtightening and thought I'd seen various mentions of PTFE tape on these on the forum.
Weep from the main hose is still there and I don't want to risk tightening further so I think I'll drain the brake and take the hose off - give it a good clean along with the caliper inlet and see if that cures it.
Thanks again,
Andy
McCabe-Thiele (Ted):
--- Quote from: TrickyMicky on April 23, 2021, 07:03:23 AM ---Use of PTFE tape on domestic gas fittings - OK. Max pressure of domestic gas is 60PSI
Use of PTFE tape on domestic mains water - OK. Max pressure of domestic water is 80PSI
Use of PTFE tape or any other sealant on hydraulic vehicle brakes is a no-no! All fittings must be assembled dry. The 'lock-up' pressure in a non-servo assisted brake pipe is approx. 900-1000PSI. The metal brake pipes have to be manufactured to withstand a test pressure of 5000PSI. Leave the PTFE tape in the drawer in the cupboard with the plumbers fittings.
--- End quote ---
My understanding of most brake fittings is that the convex & double convex type fittings rely on compressing the end of a brake fitting into the mating surface so no sealant or tap should be used. New steel pipes that have not been fitted perviously will in some cases require some extra pressure from the fitting to meet up with the mating surface to form a new seal. So I agree with you.
I've used PTFE tape on taper threads such as a plug. My belief is that in these circumstances the PTFE tape film acts like a thread blocker/locker so the comparison between say water pressure & brake pressure might not apply. I've not used on a bike but on my various cars I have used the Kunifer (Copper based) type brake pipes as they are easier to form- I find they do not require the same pressure to tighten a union even with new pipes compared to all steel ones. Interestingly MB only supply new all steel type brake pipes as authorised fittments to the dealer workshops.
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