ztim

Does the diaphragm in the brake fluid reservoir do anything more than act as a gasket? Mine wants to pop the cover off. I don't have a wire clasp to hold it down. thanks

Oldsguy

You need the wire clasp. If the lid doesn't stay tight then the fluid will become contaminated from moisture in the air (or airborne dust, etc.). Brake fluid is hydroscopic (sp?) it has an affinity for water. That diapharm is a gasket and is shaped like an accrodion to take up the spare space in the resivoir as the fluid level drops down. I don't know if you can buy just that wire clasp or not but it is worth a try, otherwise you will have to buy a whole new master cylinder.

ztim

Ok. Thanks. The master cylinder is the kind without a clasp. All it has is four hook-like things that snap over the cylinder's corners.

Oldsguy

Oh ok, I am not familiar with that. The wire clasp (like a bail handle) is the one I thought you had. Are those four things on the corner supposed to hold the top down tightly? If they are supposed to and aren't, you are still in a situation and I don't know what to recommend.

ztim

Yes. They are supposed to hold it down tightly. I'll do some more checking around. If I have to I guess I'll get a new master.

Mister

If your master brake cylinder looks like the picture attached, where the little plastic snaps are part of the lid, then you can buy a replacement lid from NAPA. It's part # BK 6751201. Says it is correct for a 1984 Delta 88.

Mister

Or Autozone Part #42041.

Mister

Advance Auto Parts uses the same part number: 42041

Mister

O'Reilly Auto Parts also uses part number 42041, although it will cost you twice as much there, for some reason. More than half the price of a whole cylinder.

ztim

That is about right although there is no angle on the cylinder. The top looks right. Thanks. I use Autozone most of the time. Napa steered me wrong too many times. Their money is in the big ticket items around here.
Thanks

smcurro

The gasket diaphram under the cover of the master cylinder is very important, not only does it keep water from getting in, but it keeps brake fluid from getting out! A snug fit is essential otherwise a seemingly correct master cylinder cover can still leak. I know from experience! On the older all steel master cylinders it is a good idea to remove the wire from the cylinder and gently bend it in order to make a tighter fit if the fit is not tight already. It should not be able to be unclipped by hand. A loose cover can not only allow brake fluid to make a mess of the master cylinder and brake booster/firewall, but it can potentially allow air into the brake lines and compromise the brakes if neglected long enough. Upon covering the master cylinder for the long run, the gasket and corresponding surfaces should be made very clean and free of any rust and dirt to insure a good seal. Be careful when cleaning the master cylinder and make sure not to drop any dirt into the brake fluid. The best time to clean the master cylinder gasket edge is right out of the box before it is installed. Even brand new these surfaces can be cruddy and uneven, especially on older all steel master cylinders. Brake fluid strips paint and my freshly painted brake booster was ruined due to this leaky seal - I had to take it out again and repaint it!

ztim

I bought both the cover and the gasket only to find out I didn't really need them after all. I had been laying in the gasket and the trying to put the cover on. I should have put the gasket on the cover in the groove and then put it on the master cylender. I'm learning.

smcurro

Either way should work but it may be easier to put the gasket in the cover first like you said. Plus at least now you have a spare cover in case the old one ever cracks or whatever. Be sure to thoroughly wash your hands as soon as you're done, brake fluid is some nasty and poisonous stuff.

Oldsguy

Just as a sidenote. On my '46 and I am sure quite a few others that are older, the cap is steel and threads onto the cylinder, no gasket.

smcurro

yeah that's good point. I have a 70 cutlass and it has casted steel reservoirs and a stamped sheet metal cap. I've always seen some variation of either mine or the plastic reservior type on every gm car from at least '68 on. It's very important to make sure nothing can get in or out of the reservoir on any hydraulic brake system irregardless or year, make or model. Same goes for hydraulic clutches. Theres nothing I hate more than brake fluid leaking all over the place and making an awful mess!

bonesbmx

hey guys
i was wondering how does a master cylinder fail? today on i push on the pedal and it goes to the floor when i pump it its fine and firm but every so often it pushes down to floor again. i happen to have new pads drums and brake cylinders for both front and back so i installed them. pedal is firmer but still every so often goes to floor i gotta pump them to get it back. i took to brake shop they blead lines and told me the master is going. just wanted to hear from you guys first. everything is new even the lines so it would have to be either cylinder or booster right?
thanks

smcurro

I can't tell you how master cylinders work or how they fail, but I know for some reason they really aren't an expensive part. A replacement master cylinder should be around 25 dollars - at least that's how much mine cost me (for a 1970 cutlass, 1968 skylark and a 1979 camaro). Whenever I have to work on brakes on a car for whatever reason, I always change the master cylinder if I'm unaware of its age. It's some pretty cheap insurance and saves you one BIG headache. I don't know of any way to test a master cylinder; they're usually pretty nasty anyways so I just chuck them and paint the new one. Theres very little additional work to replace the master cylinder if you're already going to bleed the lines. Now, as for the brake booster, those are a bit more work to change but can be tested easily. The easiest way to detect a bad booster is if the pedal becomes immediately hard to push once the car is shut off. A brake booster in full working order should have vacuum leftover after the car is shut off for 1-3 full pedal extentions. The best way to definitively test if the booster is still any good is to test it with a vacuum pump. A vacuum pump is a niftly handheld pump with a gauge attached that tells you how much vacuum you are creating. A standard vacuum pump kit comes with all sorts of adapters that you will need to use to adapt to the vacuum inlet hole on the left top of the brake booster. My vacuum pump is made by a company called mityvac and I find it is a very useful and valuable tool. Adapt the pump to the brake booster and start pumping! Be sure to have a tight seal throughout when using the vacuum pump. A standard brake booster holds a good amount of air but you will see on the gauge that you are slowly building up vacuum. If after many strokes you cannot build up any vacuum or if the booster does not hold the vacuum you have built up then your brake booster is no good. You will have to buy another as I have never heard of these things being able to be fixed. The only company I've ever heard of that makes replacement boosters is a company called cardone, and their products are readily available at autozone. An interesting occurance I noticed is at autozone it is actually cheaper to buy a brake booster AND master cylinder together from cardone than a brake booster alone, but this is of course only for the cars I've worked on. The brake booster and master cylinder should run you about 90 dollars. (Again, this is how much it was for a 1970 cutlass, 1968 skylark, or 1979 camaro.) Now, you will notice the brake booster is between the master cylinder and firewall. It is indeed possible to change the booster without opening the brake lines to the master cylinder but this must be done VERY carefully. If you feel you can't do this go ahead and take off the brake lines; this is better than accidentally breaking them. The first step is to get under that dashboard and detach the connecting rod from the brake pedal. Secondly, remove the master cylinder from the brake booster. Finally, the brake booster is attached to the firewall with four studs pressed into the firewall. Be sure the studs do not spin in the firewall and chew out the metal when removing the nuts. Some time or another you should have noticed that the vacuum line to the brake booster has a check valve on it. If you remove the check valve from the hose you can test it with that handy vacuum pump. When the vacuum pump is attached to the engine side of the valve, it should hold zero vacuum and just suck through. When the vacuum pump is attached to the booster side, it should hold vacuum indefinitly and not leak any vacuum. If the check valve fails these tests then go ahead and replace it, there should be a new one provided with the new brake booster. The rest of the deinstallation/installation is pretty straightforward but don't hesitate to ask me any questions you may have. I have done this procedure three times with a 100% success rate. It sounds like you most likely need a new master cylinder and not a new brake booster, but go ahead and check the booster and the check valve if you have a vacuum pump. The booster is a lot easier to change while changing the master cylinder than by itself.

bonesbmx

thanks after playing around i figure that it is the master cylinder i still have good pump but i will most likely change both

ztim

But how can the master cylinder go bad? It seems there isn't much to them.

bonesbmx

from what ive been reading theres a small piston inside. in my book it says when reinstalling fill resivoir with fluid then to slowly push the pistons into the master cylinder with a screw driver do this untill air bubbles stop. mabey when it breaks the piston stops moving?

smcurro

I think the piston wears out and begins leaking fluid past the seal on the piston shaft inside the cylinder. I remember there is a very specific procedure for getting the new cylinder ready to be installed but I really don't remember what it is. You have to bleed the master cylinder outside of the car before installing it. Everything should be thoroughly explained in the directions that come with the master cylinder.

ztim

Thanks. I need a shop.

tarps3

Hi Dan,
I have a '53 that also has a screw-on cap/dipstick.
Can fluid leak out of this cap if is is overfilled and pressure is applied?

I have a leak after I rebuilt my master cylinder.
The MC itself is perfec and doesn't leak, but I seem to have a leak where the brake line connects to the MC OR at the dipstick tube...

Casey