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3/4 brakes arn't bad,wait what?

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So I am replacing my brake master cylinder on my 81 hatch offroader. No matter how I tried to bleed the brakes I would always loose my brakes after a short while. The last time I bled the brakes before something clicked in my head I managed to get 3 outta 4 brakes working at least for a little bit then back to nothing.So anyway I take out the old one with fluid in the reservoirs still and press in the plunger.Nothing comes out of the "F" port on the MC but fluid squirts out of the "R" port.The car has a diagonal split as I am sure you all are aware of. LF to RR kinda thing.So how exactly did I get 3/4 brakes working when only 2/4 should have worked at all?

 

I'm just a bit stumped on this.If it makes any difference It was one of the rear brakes that didn't work.

 

also my car originally came with a 13/16 MC and rear drums.My front brakes were leaking from the handbrake assembly so I replaced those and did the rear disk conversion at the same.It is worth noting that I have EA82 front suspension/brakes on my 81.My question is the replacement MC is 3/4" bore with the bleed screws on opposite sides as the old one.No big deal.I am just wondering how that smaller MC will affect braking.I know that smaller MC's increase line pressure at the expense of needing more travel to displace a similar amount of fluid, so how will that work with the EA82 vented rotors up front and disks in the back.

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You should be using a bigger MC so the volume is correct for the system. It sounds to me like you have two problems. First, the MC is toast. It's internally bypassing so you're getting little to no pressure on that circuit. Second, I'd say that the rear caliper that isn't working is binding. It could be the piston or the slides. If the piston is sticking, what little pressure the MC is developing on that circuit is going to the opposite front caliper giving the impression of working.

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He said the replacement was smaller. What I meant was he should be using the 13/16. How he got a smaller one, I'm not sure. Of course, the aftermarket world is full of oddities. Volume is pretty important when changing wheel cylinders to calipers. It's a good thing that Subaru over engineered a lot of things.

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After getting everything bled I think I need to get a new MC with at least 13/16 or better.The 3/4" seems to bottom when I get about 5/8 braking effort.A full on "panic stop" stops the car like I am just using the handbrake.Which is less than stellar with 4 wheel disk brakes.

 

not sure why rock auto sent a 3/4" instead of 13/16"..

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After getting everything bled I think I need to get a new MC with at least 13/16 or better.The 3/4" seems to bottom when I get about 5/8 braking effort.A full on "panic stop" stops the car like I am just using the handbrake.Which is less than stellar with 4 wheel disk brakes.

 

not sure why rock auto sent a 3/4" instead of 13/16"..

 

You got a 3/4 one for an 81 brat,not a 13/16 one for an 81 hatch.

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the smaller the bore the more psi it creates the biger the bore the more volume it makes on the race buggy i whent from 1-5/16 to a 3/4 to increase the preshure. The biger one could not hold the car back but the smaller one did. I do not think the size of master is that important. Is it posibble that has a seized wheel cly or addjusters in wrong or air traped somwere

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