Spongy brakes when hot

[Kerplunk]

my new to me 1963 Polara needed a rear flex line and rear shoes (lots of lining left but starting to separate) did all that bled the brakes but now if I drive the car for half hour or so the brakes get spongy. It's feels ok when it's cold. I've double checked the drums and they look good and adjusted ok. The previous owner upgraded to front disks. Any ideas this one has me stumped.

 

[bm02tj]

Moisture in the brake fluid
or a bad hose that is expanding

 

[440+6]

Did you bleed the fronts also? Front hoses OK? Master cylinder crapping out?
Moisture in the lines, and especially in the calipers will boil.

 

[Kerplunk]

Thanks guys I took it for a test drive and found the right rear wheel super hot so that must be where I'm getting the boiling fluid from. I'm guessing somethings hanging up even though the drum come off easily enough. I guess I should take it apart and make sure everything is ok put it back together and test.

 

[bm02tj]

Did you bleed brakes with a sealed bottle of brake fluid or one you had on the shelf
Brake fluid absorbs moisture from the air

 

[Kerplunk]

It was a sealed bottle but I'm not sure how long the rest of the fluid has been in the car. Probably a good idea to flush it.

 

[Ranger16]

Probably a dumb question, but are you using DOT3 fluid?

 

[Kerplunk]

Yup dot3 if I replace the fluid should I go with dot3 I've read that dot5 is good but is it compatible with our 50 year old brakes?

 

[Ranger16]

I'd stick with DOT 3.

Here's what Wikipedia has to say about DOT 5.

"DOT 5 is one of several North American designations of automotive hydraulic brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.


DOT 5 is a silicone-based brake fluid (contains at least 70% by weight of a diorgano polysiloxane[1]).


Unlike polyethylene glycol based fluids, Dot 5 is hydrophobic.[2] An advantage over other forms of brake fluid is that silicone has a more stable viscosity index over a wider temperature range. Another property is that it does not damage paint.[citation needed]

Using DOT 5 in a DOT 3 or DOT 4 system without proper flushing will cause damage to the seals and cause brake failure.[citation needed] DOT 5 brake fluid is not compatible with anti-lock brake systems. DOT 5 brake fluid absorbs a small amount of air requiring care when bleeding the system of air."

 

[funknut]

Never mix DOT 3/4 (glycol) with DOT 5 (silicone).

There is DOT 5.1 which is glycol-based and therefore compatible with DOT 3 and DOT 4.

As long as you're changing your brake fluid at regular intervals, IMO just stick with what you have.