Caledonian Chipmunks



Author Topic: Brake Dilemma  (Read 463 times)

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Offline yakxx

  • Posts: 171
  • Country: pt
Brake Dilemma
« on: Sunday September 16, 2018, 09:18:39 UTC »
Having just completed a 2500 mile wonderful Chipmunk trip I experienced a rather strange brake issue and before I tear out the whole system I would welcome any comments ... ( I also had a trim cable jam or break which meant the last 900 miles without trim control...just manually set to neutral....it works pretty well ) ....back to brakes:   It happened twice and both at the same airfield in Spain which happens to be 3000ft and to taxi to the pumps takes at least ten minutes of zig,zagging....as I reached the pumps and applied left brake I had absolutely no pressure....had to hit mags off and hit the right brake  which of course swung me around the other way just missing the pumps and the fuel man who thought I was insane....I say it happened twice because on an earlier visit the same  brake became soft but workable , but this time I had absolutely no pressure...I pumped and pumped..nothing, then after about ten minutes pressure started to come back and then it was normal...I then flew another 500 miles stopping many times without issue.....OK....it would seem it may have overheated but why zero pressure....If the pads had stuck in or out....I should still have pressure....If its the master cylinder why would it happen and then come back.....both times after a long taxi....and at 3000 elevation.   Brake fluid tank is full....I can only think that its a seal issue in the master...but why would the brake come to life after sitting stationary for ten minutes........air bubbles in the line boiling like an F1 car.....dont think so.....naturally I will be replacing the total system , seals  ....but would love to hear any comments....
On the trim cable.....I haven't even taken a look yet.....after two solid weeks flying, I dont want to see her.....for a few days

Offline Bob

  • Posts: 348
  • Country: 00
Re: Brake Dilemma
« Reply #1 on: Monday September 17, 2018, 21:04:42 UTC »
I'd go for boiled fluid due altitude/temp.I take it they are the original Dunlops?

Offline speedbird1

  • Posts: 545
  • Country: us
Re: Brake Dilemma
« Reply #2 on: Tuesday September 18, 2018, 00:27:39 UTC »
A friend here in the US just had a similar issue when he had to taxi a very long way, with a crosswind, at a major airport.  5606/OM15/DTD585 will do this when it gets hot and Dunlop brakes don't help either.
Speedbird1

Offline yakxx

  • Posts: 171
  • Country: pt
Re: Brake Dilemma
« Reply #3 on: Tuesday September 18, 2018, 16:48:47 UTC »
Yes, very similar....big crosswind and at least ten minutes taxi....

My system has always used DOT 4 ( Car fluid) ...for the last 25 years...so I dont want to change to Red aircraft fluid now unless I change all the seals right through.....

thanks for your comments ....

Offline Dick Gower

  • Posts: 301
  • Country: au
Re: Brake Dilemma
« Reply #4 on: Tuesday September 25, 2018, 07:05:29 UTC »
Doesn't car fluid absorb water over time and this lowers the boiling point?  A friend of mine had this happen in a car until he learned not to drive like Fangio.
Dick Gower
Melbourne,
Australia.

Offline speedbird1

  • Posts: 545
  • Country: us
Re: Brake Dilemma
« Reply #5 on: Tuesday September 25, 2018, 15:38:42 UTC »
So does OM15/DTD585/5606.  ie: the Red stuff, absorbs water as well.
I have seen several stuck pistons in accumulators that don't get operated very often and it is always corrosion where the water sat under the piston and its seals.  Also the accumulators were mounted horizontally.
Speedbird 1.

Offline Nigel Stevens

  • Moderator
  • Posts: 245
  • Country: fr
Re: Brake Dilemma
« Reply #6 on: Friday September 28, 2018, 11:56:32 UTC »
Agree sounds like boiling brake fluid. Car brake fluids do absorb water over time. Most high performance cars now  have recommendations to flush the old brake fluid out of the system with new fluid on either a two or four year cycle. This may apply to low performance cars as well!!!!   Pressure pump systems are avaialable which fit on the car master cylinder and then force the new fluid through until clean new fluid comes out of the purge fittings. Unfortunately they are too large to fit on the Chipmunk MC without some modifications, but flushing new fluid through should cure the problem.
Nigel

Offline rod brown

  • Posts: 2
  • Country: gb
Re: Brake Dilemma
« Reply #7 on: Saturday October 13, 2018, 22:58:21 UTC »
Nigel has hit the problem head on and best suggestion would be to change the fluid for some new DOT
4. Chances are that it will be old and dis-coloured. Having read about brake bleeding problems here
on Cal Chips some time ago I found it best to draw out the old fluid from the reservoir, fill up with new fluid and bleed one side at a time until new fluid flowed through.
Research shows that DOT 4 has a temperature range a bit higher than the Shell Type 41 which has
replaced DTD 585 here in the UK so not much point in changing the fluid type. There is some good
information available from Shell by typing DTD 585 or  into Google.

Offline devonchip

  • Posts: 5
  • Country: gb
Re: Brake Dilemma
« Reply #8 on: Sunday October 14, 2018, 16:30:50 UTC »
We have had a similar issue that turned out to be the brake push rod jamming against the master cylinder circlip tags.  We had all the symptoms of brake fade/ fluid
issues etc. Brakes would not bleed because the piston was not returning fully.

Offline p51aba

  • Posts: 2
  • Country: gb
Re: Brake Dilemma
« Reply #9 on: Saturday October 20, 2018, 23:14:26 UTC »
If this problem IS due to DTD585 (Fluid 41 / OM15) boiling during extensive taxiing , maybe a change to AeroShell Fluid 31 would help? This is a synthetic super clean hydraulic oil which has replaced DTD585 (Fluid 41 / OM15) in the RAF on C130 Hercules aircraft. The C130K model utilised DTD585 (OM15), but the newer J model utilises OX19 (Aeroshell Fluid 31). Fluid 31 has a useful top temperature in excess of 200 degrees C, where as DTD585 (OM15 / Fluid 41 only has a top temperature of 90 degrees C.

There was initial concern with hydraulic components having previously been used within an OM15 environment suffering from 'Seal shock' when utilised in an OX19 environment. To my knowledge, this was completely unfounded but completely justified concern.

There would of course be a requirement to submit this as a modification to the LAA or CAA depending on whether you operate on a Permit or CofA.

I would like to add some useful notes on bleeding Chipmunk brakes that I have used for many years:
1. Prior to bleeding, gain access to the master cylinders through the lower access panel and ensure the pistons have fully returned to their off position.
2. Bleed the brakes from the calliper bleed nipples up to the reservoir. A simple pump to force the fluid through the calliper is required and a plastic bag around the reservoir to catch the expelled fluid.

The above process will bleed the brakes on both sides within 10 minutes, to a solid non spongey condition. This was devised after spending hours trying to get a satisfactory result bleeding the brakes using the process from the AMM after refitting the undercarriage legs post X-Ray of the fittings.

Offline john henderson

  • Posts: 273
  • Country: gb
Re: Brake Dilemma
« Reply #10 on: Monday October 22, 2018, 15:24:48 UTC »
We bleed from the caliper up, using a few psi of compressed air to force fluid from a filled reservoir on the ground. Works well and is quick. No pumping required.  Can be done by one person.  Same rules apply about retracting pistons fully.
john h

Offline Dick Gower

  • Posts: 301
  • Country: au
Re: Brake Dilemma
« Reply #11 on: Wednesday October 24, 2018, 04:31:24 UTC »
We have had a similar issue that turned out to be the brake push rod jamming against the master cylinder circlip tags.  We had all the symptoms of brake fade/ fluid
issues etc. Brakes would not bleed because the piston was not returning fully.

There is conflicting information between two publications (Dunlop and MoD) as to the orientation of these circlips in order to avoid that jamming problem Devonchip.
The Dunlop manual is correct.
Dick Gower
Melbourne,
Australia.

 

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