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Rear brake pressure loss

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Bike - 02 CRF450R, I just got the bike, the back brake pads were like metal to metal so I had to replace the brake pads right away.

When I press down on the rear brakes strongly, the pedal is firm and the brakes work as long as you don't release some of the downforce on the brake pedal. If you push down on the brake pedal slowly or without as much force, the brake pedal will continue to go all the way down until the brake pedal is all the way down and then the brakes don't work at all. So if I'm goin down a hill and using the back brakes, they will stop working unless I am pushing down as hard as I can to lock up the rear wheel. So I have to pump the brakes on and off and they will work, but I can't hold them on or else they stop working.

I've read through the forum but never heard of anyone talking about the same exact problem or desribe what is happening like what is happening to my brakes. I've heard about rebuild kits for the master cylinder fixing brake problems, and new brake lines fixing problems. I'm thinking there must be a bad seal inside the master cylinder or the caliper, but I started this thread to try and get your thoughts from your brake problem experiences before I go spending money.

Anyone else experience the same issues with the brake pressure loss and what was the fix? I started bleeding the brakes but I really don't think they need bled because the brakes work properly when they are pressed down firmly. I'd appreciate any help. thank you!

I have had a 02 CR 250 for the past 2 years, all the brake parts look exactly the same on both bikes, I've never had any problems with that bike's brakes except replacing the pads of course. I know I could switch the master cylinders or the calipers between the 2 bikes to try and figure out which part is causing the problem, but then I would have to bleed the brakes again on both bikes. Asking on here first is easier.

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i would bet that you brake line is bad. if its not the brake line the master cylinder would need to rebuilt. i would bleed the brakes and change the fluid before spending any money. you may get lucky

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I was thinking it wouldn't be the brake line, I don't see how it could be the brake line if the brakes pressure is held good when you push on the brake pedal very hard. It just doesn't make any sense to me that the brake line pressure will hold okay when pressed hard but the brake system doesn't hold pressure when the pedal is pushed down slowly without much force. thanks for the reply.

I can't remember exactly but I think I had the same problem on my old 89 YZ 250, to fix that problem I just bought a complete used brake system on eBay that worked good. I think it was the master cylinder that was bad on that bike.

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Are the calipers in these brake systems pretty good, so when there is a problem it is either the master cylinder or the brake line?

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Are the calipers in these brake systems pretty good, so when there is a problem it is either the master cylinder or the brake line?

sometimes these lines can be a :ride::applause::thumbsup::applause: to bleed... bleed it several times to make sure there is no air in the line. if you are certain there is no air then either time for a line or master cylinder rebuild!

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I started bleeding it, then I decided to ask here. I will continue to bleed it but I doubt that will fix the problem. Wouldn't the brake pedal not be firm when pressed down very hard if there was air in the system? That's why I doubt there is air in there.

Could this problem somehow at all be caused from the old brake pads being worn completely down to metal to metal, then the pads being replaced, the caliper piston was like all the way out then it needed pushed all the way back in to fit the new thicker pads. I only breifly rode the bike once before changing the pads, and I can't remember if I noticed the brake problem before or after I changed the pads.

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the brake lines on the earlier crf450 seem to fail and give the symptoms you are giving. i had very similiar symptoms with my 02 and i rebuilt the master cylinderand caliper, but still had the same problem. when i changed the brake line it was fixed. if you watch you brake line while you push the brake petal i bet you will see it swell or bulge. if you can see it swell that is what is causeing your problem. pick up a stainless steel braided line for about $30 and your problem will be gone.

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I do not see the brake line swell or bulge anywhere when pushing down on the brake pedal with all different forces, fast, slow, strongly or not. It would have to bulge when I press down on the brake slowly if that was the problem, because the brake loss problem only happens when the brake pedal is pushed down slowly without a lot of force. You would think the brake line could only bulge when pushing down on the brake pedal with alot of force and when their is a lot of pressure in the system. There is not any brake fluid pressure in the system when I push down slowly which is when the brakes don't work.

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The first thing you should do is go back and take the new brake pads completely out and verify that the front tangs of the pads are really in the correct place,sometimes I have seen people get these tangs slightly out of position and this will cause the symptoms you describe.It kind of holds one of the pads at a very slight angle.I however don't really think this is the problem check the rear brake pin,the one that retains the pads for any groves and replace if not perfect.Probably your master cylinder upper seal is breeched and you are only operating on the lower seal.cheers

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That's not the problem (brake pads), I removed the caliper from the bike (brake line still connected) to hold it up higher in the air while bleeding the brakes and the problem is the same, and lots of bleeding didn't help. It is probably the master cylinder. thanks.

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Best bet is to change your brake fluid. Like engine oil, it evenutally wears out.

That would not change anything, maybe you didn't read that the brakes work good when you press on them firmly, but press on the brakes slowly and pedal drops all the way down and they don't work.

I'm not a brake fluid expert, but I don't see how changing the brake fluid would not allow my problem to happen. With all the bleeding I have done it probably is all changed now.

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buy a mastercylinder rebuild kit, if it was the brakeline you would't be able to lock up the rearwheel like you describe. also only use dot4 fluid, Ive tried dot 5 DON'T use that!

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Thanks. I was putting in dot 3 when bleeding because that's all I had, didn't want to go get dot 4, and I read online the only difference or main difference is the boiling point. should I definately switch to dot 4 ?

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