Discussion Forum—A Way with Words, a fun radio show and podcast about language

well,
plead is Latin based
(from Anglo-Fr. pleder, O.Fr. pleider, plaidier, “agreement, discussion, lawsuit,” from M.L. placitare, from L.L. placitum)

while,

“lead” whose past tense is “led” appears to be rooted in Goth
(from W.Gmc. *laithjan (cf. O.S. lithan, O.N. liða “to go,” O.H.G. ga-lidan “to travel,” Goth. ga-leiþan “to go”).

“bleed” whose past tense is “bled” is also Germanic
O.E. bledan, from P.Gmc. *blothjan “emit blood” (cf. O.N. blæða, Ger. bluten)

“breed” whose past tense is “bred” is also Germanic
(from W.Gmc. *brodjan (cf. O.H.G. bruoten, Ger. brüten “to brood, hatch”),

"read" whose past tense is read pronounced red is Germanic
from P.Gmc. *raedanan (cf. O.N. raða, O.Fris. reda, Du. raden, O.H.G. ratan, Ger. raten "to advise, counsel, guess"),

I'm guessing the past tense exception (not using the regular “ed” suffix)
has it's origins in Norse and German,
and since “plea” has Latin roots, it should be subject to the regular past “ed”.

though
the internet tells me both pled and pleaded are acceptable.
and
"heed" whose past tense is "heeded" is also Germanic
from W.Gmc. *hodjan (cf. OS. hodian, O.Fris. hoda, Ger. hüten "to guard, watch")

http://www.etymonline.com/index.php