TRX300FW 1991 - Front Brake Bleed

[shreveportarchitect]

I’m rebuilding a 1991 trx300 fw.  The brake master cylinder was dry/empty.  Hoses appear to be clear.  Bleed valves are clear.  I replaced the master cylinder.  I cannot get the fluid to flow out of master cylinder when pumping handle.  

I am doing everything per the videos I have watched on YouTube.  I can get a few bubbles out of the hose at the end of the bleed valves but the fluid level in the reservoir never goes down.  I put my thumb over the outflow with the hose off, squeeze handle, and get some back pressure thru the oil seal in the cup and can feel some pressure and suction on my thumb

I'm at my wits end and see why brake bleeeding is reserved for the seven levels of hell.  Any suggestions?

[davefrombc]

There is very little volume  per stroke on the  master cylinder , so  I would suggest  just  keep  bleeding the system  until  all  the air  bubbles out.  A  faster way , but  a lot more expensive is to  buy a  tool to  do  a reverse  brake bleed.  It's a handy tool  if you  need to vacuum  test switches and  air  motors  as well  as  bleed brakes  on any of your vehicles. Here  is  one  on  Amazon.com:

 

https://www.amazon.ca/Performance-Tool-W87030-Automotive-Bleeder/dp/B013VUZ7WG?th=1&psc=1&source=googleshopping&locale=en-CA&tag=googcana-20&ref=pd_sl_54ilvkye00_e

[shreveportarchitect]

If I disconnect the hose from the master cylinder and pump the handle, should I have flow out of the opening?

[davefrombc]

You should.. I  don't know  if you're familiar with bleeding  brakes ,  so  I'll  toss this in .  When  you  pump,  while the  handle  is down in the pumped position,  close the brake bleeder vqalve so the   master cylinder  has to  draw from the reservoir rather than  drawing back from the  line . Same  goes when  just  at the   master cylinder itself ..  block the outlet with your thumb when  releasing the   brake handle .  You should  get pressurized  fluid on each pump  after the first .

 

[shreveportarchitect]

I’ve done that.  So, when I have the lid off the master cylinder, thumb over the hole and squeeze, I get blowback thru fluid thru the  seal at the bottom of reservoir.  Should I have that?

[shreveportarchitect]

...and you can talk to me like I’m an amateur, because I am.  No offense here.

[davefrombc]

No, there should be no  blow  back into the reservoir when  you  put  pressure on the pump  , only on release   should any  air blow back, and only  until the  air is out of the  pump. it sounds like the plunger on the  master cylinder has a defective seal. There  is no  check  valve on the  master cylinder so  when  you release the brakes the  wheel  cylinders push the fluid on the  lines  back to the reservoir. If there  was a check  valve ,  once the brakes were applied  they  could not release.  I think you got a defective  master cylinder. There  are videos on rebuilding them   on Youtube and rebuild  kits are  not expensive. 

[shreveportarchitect]

Hmmm.  I just rebuilt it with a new piston.  I wonder if I have the o-ring seal on backwards.  I found a cheap hand pump at harbor freight I'm going to try.  I'll let you know.  Thank you for your help.

[davefrombc]

The   edges of the  U   cup seal(s) go towards the outlet. The  pressure   in the system  when  you  press the brake forces the lips of the cup  out  to  hold the pressure. If it  is  backward the  pressure pushes them  in   and bypasses  the  piston. That  easily could be the problem . You wouldn't be the first to accidentally  put them  in  backwards 😇