Loosing brake pressure in my 71 Skylark

Got factory power/front disc rear shoe brakes in my 71 Skylark Custom. All of a sudden I'm loosing pressure as I come to a stop, pump it up and it's OK then I loose pressure again. No fluid leaks and brakes are OK all around. Checked Master Cylinder which has new cap and there is no loss of fluid and no visible leaks. Replaced the check valve on the brake booster a while back and everthing external seems OK. More like a vacuum/air pressure leak than fluid. What gives??

 

If it was vacuum it would have a hard pedal, suspect the master. fluid could be slipping by the seals. look for leak inside the car.

 

If you have a rebuilt master....trash it.
Buy a new master.....rebuilds are known to have poor pressure(cycles fluid between front and rear reservoirs instead of flowing to brakes)

 

Can't find a fluid loss anywhere! Inside or out!

 

You wont find a leak....it is cycling/bypassing internally.

GO buy a NEW master.

 

"Can't find a fluid loss anywhere! Inside or out!"

Like George said, if the master cylinder cycles fluid between front and rear reservoirs instead of flowing to brakes then you won't have an external leak because the fluid is circulating within the master cylinder.

The fluid goes past the piston, so the pressure doesn't have a chance of transferring down the lines to the brakes.
Duane

 

Thanks guys

Is it safe to say I should get a new one vs. a rebuilt one? If new is the answer where's the best place to go? I'm in Clermont Fl and have access to Napa, Auto Zone, Advance Auto etc.

FYI

When I first bought the car I had two rear wheel cylinders that were rusty fluid bad. The master looked like the fluid was from 71. That said I replaced the master cover, cleaned the master up, flushed new fluid through the sytem and replaced the leaking rear wheel cylinders. Do you guys still think it's just the master or might I have a hidden problem??

 

Thanks,

Used the one man brake bleader system and pulled lots of fluid through each wheel to get all the old crap out! Never let the master run low and all was OK for awhile! Thought I had it fixed but obviously I was wrong! Will get a new one and start over.

 

not to hijack your thread.. but I have been fighting a braking problem for awhile and just wondering if a bad MC could cause random uneven braking?

 

Going through this on my GS400 convertible, just replaced all 4 wheel cylinders and shoes, and a couple of the lines. Went to bleed it, and felt like I was getting pressure. Drove the car half a block and pedal went completely to the floor, after a couple of pumps, had "some" brakes, but not much, I figure I need to replace the master cylinder too!!! Now, here is the 64,000 dollar question. When I go to get my new master at O' Reileys, how do I tell if I'm getting a new one or remanufactured?????

 

If you ask them when you are buying it they will tell you. This also might require you to order the new part, if not in stock. I also had problems with low peddle height after new wheel cyls, master and shoes. Don't overlook simple things like your brake adjustment.

 

Got 3 three choices for a new master cylinder for my 71 Buick.

1. Cardone Select from Advance Auto Parts with a lifetime warranty
2. Bendix with a one year warranty from Advance Auto
Parts
3. Fenco from Auto Zone with a lifetime warranty.
Nappa was closed!
Has anybody used these brands? Any opinions? They all seem to be made in China!

 

the important thing is to bench bleed it correctly and get all the air out of the system. you can install a re-manufactured one and if you install it right it will work better than a brand new one that is installed incorrect.

 

Check http://www.rockauto.com/ they have a number of choices, and with them you are going to get the right part. I have pretty much quit going to parts stores, I have much better luck with rockauto.

 

I have power drum all the way around, which one of these from Rock Auto do I need?

<TABLE class=main style="BORDER-RIGHT: 2px groove; BORDER-TOP: 2px groove; BORDER-LEFT: 2px groove; BORDER-BOTTOM: 2px groove; BACKGROUND-COLOR: white"><TBODY><TR><TD class=center style="FONT-WEIGHT: bold" colSpan=5><TABLE class=layout><TBODY><TR><TD class="left closebtn"></TD><TD class=center style="WIDTH: 90%">1968 BUICK GS 400 6.6L 400cid V8 : Brake/Wheel Hub : Master Cylinder </SPAN></TD><TD class=right>Related Parts</TD></TR></TBODY></TABLE></TD></TR><TR><TH>Item</TH><TH>Price</TH><TH>Core</TH><TH>Total</TH><TH></TH></TR><TR class=part0 id=table298part0><TD><TABLE class=parts><TBODY><TR><TD class=left><TABLE class=layout><TBODY><TR><TD>RAYBESTOS Part # MC36306 {Professional Grade Bore Size=1 1/8" Number of Ports=2 Primary Outlet Size=1/2x20 Secondary Outlet Size=9/16x18}
Front Disc Brakes; Rear Drum Brakes; Power Brakes</TD></TR></TBODY></TABLE></TD><TD class=right style="HEIGHT: 40px">
</TD></TR></TBODY></TABLE></TD><TD class=center id=node298part0price>$22.79</TD><TD class=center id=node298part0core>$0.00</TD><TD class=center id=node298part0total>$22.79</TD><TD class=center>
</TD></TR><TR class=part1 id=table298part1><TD><TABLE class=parts><TBODY><TR><TD class=left><TABLE class=layout><TBODY><TR><TD>A-1 CARDONE Part # 101521 {[Master Cylinder] Reman.}
With Disc Brakes</TD></TR></TBODY></TABLE></TD><TD class=right style="HEIGHT: 40px">
</TD></TR></TBODY></TABLE></TD><TD class=center id=node298part1price>$16.50</TD><TD class=center id=node298part1core>$12.00</TD><TD class=center id=node298part1total>$28.50</TD><TD class=center>
</TD></TR><TR class=part0 id=table298part2><TD><TABLE class=parts><TBODY><TR><TD class=left><TABLE class=layout><TBODY><TR><TD>A-1 CARDONE Part # 101489 {[Master Cylinder] Reman.}
With Drum Brakes</TD></TR></TBODY></TABLE></TD><TD class=right style="HEIGHT: 40px">
</TD></TR></TBODY></TABLE></TD><TD class=center id=node298part2price>$23.99</TD><TD class=center id=node298part2core>$9.00</TD><TD class=center id=node298part2total>$32.99</TD><TD class=center>
</TD></TR><TR class=part1 id=table298part3><TD><TABLE class=parts><TBODY><TR><TD class=left><TABLE class=layout><TBODY><TR><TD>RAYBESTOS Part # MC36235 {Professional Grade Bore Size=1" Number of Ports=2 Primary Outlet Size=1/2x20 Secondary Outlet Size=9/16x18}
Manual Brakes</TD></TR><TR><TD class=center>Choose: <SELECT onchange="changeOptionSelection(298, 3, this)" size=1 name=optionchoice><OPTION value=0 selected></OPTION><OPTION value=1>[Wholesaler Closeout -- 30 Day Warranty (Only 1 Remaining)] ($34.79)</OPTION><OPTION value=2>[Regular Inventory -- Full Mfr. Warranty] ($60.79)</OPTION></SELECT></TD></TR></TBODY></TABLE></TD><TD class=right style="HEIGHT: 40px">
</TD></TR></TBODY></TABLE></TD><TD class=center id=node298part3price></TD><TD class=center id=node298part3core></TD><TD class=center id=node298part3total></TD><TD class=center>
</TD></TR><TR class=part0 id=table298part4><TD><TABLE class=parts><TBODY><TR><TD class=left><TABLE class=layout><TBODY><TR><TD>ACDELCO Part # 18M13 {#18030255}
Brake Master Cylinder: Durastop; With Power Brakes; 1 CYLINDER BORE
Brake Master Cylinder: Durastop; 1 CYLINDER BORE; w/o POWER BRAKES</TD></TR><TR><TD>* Non-stock item--shipping delayed up to 3 business days</TD></TR></TBODY></TABLE></TD><TD class=right style="HEIGHT: 40px">
</TD></TR></TBODY></TABLE></TD><TD class=center id=node298part4price>$76.79</TD><TD class=center id=node298part4core>$0.00</TD><TD class=center id=node298part4total>$76.79</TD><TD class=center>
</TD></TR></TBODY></TABLE>

 

All 3 of the master cylinders I listed are new not remanufactured. The Bendix only had a one year warranty while the other two brands were lifetime.

 

a few years ago, the original master failed on my gs. i purchased a new raybestos from a company in california. had to return it as defective. a new replacement worked fine.
do NOT buy rebuilt!
the only time i would use a rebuilt m/c is if the bore/cylinder was drilled out and resleeved. this is quite common for vintage cars to maintain originality.

 

The master cylinders I referred to were being sold as new not rebuilt which cost much less. How would one tell the difference other than the computer listing or price.