Gsx 750 1980

[John W]

I recently acquired a 1980 gsx 750 to join the gsxs1100e I am rebuilding and my gs850g on the road bike.The front brakes on the GSX are really spongy and ineffective.I have bled the system with a brake bleeder,effectively changing the fluid,to no avail the lever remains spongy.The reservoir is the rectangular type and with the cover off and pulling the brake lever fluid appears to be ejected out of the small hole in the bottom of the reservoir.I believe this to to the fluid return port and should allow fluid back into the reservoir when the lever is released so as not to allow brake lock up especially when the brakes and fluid are hot.Is this correct and should fluid be ejected out of this hole when the lever is being pulled?.This to me seems to divert the flow initially from going to the calipers,thus giving excessive lever movement ,before the brakes engage.
I would appreciate some advice from the experts out there on these brakes.Thanks
John

 

[seuadr]

sounds like the seals in your master are either gummed up or failing.. i'd start there. you can take it apart and clean it up, and if it still seems spongy, look for leaks anywhere in the system, and/or rebuild your master. also check your calipers and make sure they are nice and clean inside shouldn't need to rebuild them unless one of the seals are damaged.

 

[John W]

sounds like the seals in your master are either gummed up or failing.. i'd start there. you can take it apart and clean it up, and if it still seems spongy, look for leaks anywhere in the system, and/or rebuild your master. also check your calipers and make sure they are nice and clean inside shouldn't need to rebuild them unless one of the seals are damaged.

Thanks Seuadr,makes sense to do that.Still wondering if fluid shoild be expelled thru the return port when pulling the brake lever.Any thoughts on that?

 

[Ed Vrolijk]

If i remember correctly from the last time i bled my brakes the brake fluid was expelled from the return port when i released the lever. (Wich makes sense come to think of it)
Ed

 

[John W]

If i remember correctly from the last time i bled my brakes the brake fluid was expelled from the return port when i released the lever. (Wich makes sense come to think of it)
Ed

Thats What I thought ,Ed.However fluid is expelled when I pull the the lever.So need to be sure that is not right before I go and try to rectify a non existant fault!!

 

[76FJ55]

at the beginning of the stroke the port is uncovered as the piston advances this will forve fluid up out of the hole. so you will get some back flow out of the hole as the brake is applied. weather you have more than normal I can't say, but mine will form a little gyser in the reservior if the level is low while bleeding. Another possibility is the lines. the old flexible lines becone less resistant to expansion uneder pressure with age. you may want to either try new lines or better yet stainless braided lines. it made a huge difference in the feel on my 750.

 

[John W]

at the beginning of the stroke the port is uncovered as the piston advances this will forve fluid up out of the hole. so you will get some back flow out of the hole as the brake is applied. weather you have more than normal I can't say, but mine will form a little gyser in the reservior if the level is low while bleeding. Another possibility is the lines. the old flexible lines becone less resistant to expansion uneder pressure with age. you may want to either try new lines or better yet stainless braided lines. it made a huge difference in the feel on my 750.

Mine is doing exactly as you have described,so it appears to be normal then.I have stainless braided lines on already.just removed and checked the calipers, no leaks and movement seems normal.Have to have a rethink on this one!

 

[John W]

I have noticed something odd about my master cyclinder.The service manual and the parts microfiche all show a unit with a plastic reservior,with 4 cover retaining screws.Mine has a solid metal body reservoir with two cover retaining screws.Could it be possible that this is an incorrect part that has been installed on the bike,giving the poor braking performance and spongy lever I am getting?.Any comments please?

 

[Big T]

Post a picture of it and some one will let you know

 

[hampshirehog]

You're right, an 80 should have the white rectangular plastic pot, 4 screws in the top, that comes apart from the main body of the MC. So someone has changed it, but it could be an 'upgrade' over the original.

You'll need to find out what it's from to confirm that however as it could also be from a single caliper bike and have a narrower MC piston which can show the symptoms you describe. To establish that you'll have to split the MC and measure which is a PITA so I'd try a photo first to see if someone can ID it.

Might also be sloppy original brake lines expanding as suggested earlier - stainless upgrade is money very well spent.

 

[76FJ55]

My first guess is that the plastic reservior deteriorated with age and someone replaced it with the unit you currently have. it may be the wrong piston size but you are getting controdictory results. if the piston diameter is too small you will get a spongy feeling lever but your brake will be extra powerful (low volume but higher pressure generated) if however the piston is to large you will get a very solid feeling lever but inefective baking performance (large volume displaced but at lower pressure).

 

[Suzuki mad]

Spongy feel can be down to an air leak, crap in the lines or partially seized callipers.

If you have Antidive units on the forks that can also casue a spongy feel to the brakes.

Suzuki mad

 

[John W]

Thanks for the input guys.After doing some more research,it appears that the MC is possibly from a circa 1982 Gsx 750EZ or TZ, judging from microfiche pictures.So I guess it should work!.It does make it difficult to know what to specify when ordering spares.I think I am going to look for a used orginal MC for the bike.Alternatively does anyone know of a MC upgrade for this bike?
Also twin piston calipers?

 

[seuadr]

i would imagine if it's from a similar year/model gsx it shouldn't be any worse than the stock one.. looking at the parts fishe, it looks to me just like the MC i have on my bike, which is off an 1150.. i would disassemble your master and take a look at it.. i had a MC that did something similar, and it turns out the inner part of the cylinder wall was pitted, keeping the master from getting a good seal. could be as easy as a rebuild, though, the rubber god dry and cracked and isn't sealing.

 

[John W]

i would imagine if it's from a similar year/model gsx it shouldn't be any worse than the stock one.. looking at the parts fishe, it looks to me just like the MC i have on my bike, which is off an 1150.. i would disassemble your master and take a look at it.. i had a MC that did something similar, and it turns out the inner part of the cylinder wall was pitted, keeping the master from getting a good seal. could be as easy as a rebuild, though, the rubber god dry and cracked and isn't sealing.

That will be my next step to strip the MC and examine it.if it appears u/s,then I will probably look for an orginal MC for the bike.As I am not sure which bike this MC came from it could make ordering spares a bit of a hit and miss affair.